Try the links above first, but here is my experience with those issues....
I have occasionally seen similar issues on a Mac, but rarely.
The first thing I try is to restart the machine with the card and reader attached, whatever the initialization routines are, they sometimes help.
I have seen cables be the culprit, try changing cables.
Also check the card reader manufacturer for software updates.
It also sometimes happens that an OS update breaks things that used to work

The most helpful supplier of Epoxies is Jamestown Distributors in RI. I would call them as ask.
Generally it is UV that causes yellowing. Some may have UV inhibitors. Jamestown services the boat building and craft industries. There is no more UV exposure than a boat gets, so they would know.
Epoxies work by saturating substrates then building up. Get two copies of the photo and store one just in case.
There are also polyester resins. But Jamestown would know which is best.
Good luck

I respect your curiosity, and understand the motivation to find differences between the cameras.
But there was not much that can be learned from your methodology.
The essence of a comparison is equal conditions for each and to remove as many variables as possible.
Yours was an exercise in Adding variables.
You do understand that RAW files are not an "Image Format" and thus not directly viewable, right?. You are viewing the embedded JPEG that all RAW files have, possibly changed by default parameters of your file converter.
The first step of any comparison, is understanding what you are trying to compare.

It is not my intention here to 'Pile on' negative comments, but more to say curiosity is good, but figure out how to get to some base level, so the comparison might be useful to you.
You could use this experience to design a more meaningful comparison
Good luck

You have done all the obvious things, and the lens has known issues. Short of renting from a local shop, or buying a shorter prime---if 50 would be wide enough the very inexpensive 50 /1.8 is sharp and light and a great backup to have in the bag. Having useable backup can be a lifesaver.
Given that the lens has known issues and newer versions, I would be hesitant to sink money into it.
Good Luck

OWC is a Mac specialist who carries NOS, New , used, and used Build to order. They are quite reputable

Pentax has made a bunch, I had the little Canon 40mm f/2.8 which was very nice.
Since you say Street photography -part of that tradition, especially with a Leica was Zone focusing.
If you can deal with that, just look for older film lenses in 28 or 35mm f/3.5 or 2.8 many of those are tiny by today's standards.
Whatever you choose, my advice is pick the focal length you want to work with FIRST and don't get too hung up on Pancake

You say you want to "Improve" the image from a D500 to equal that of a D850, but you do not say for what purpose. Are you trying to make a large print ? What is the specific problem you are trying to solve?
If you are simply asking 'Is there a software program that will make d500 files the same as d850 files the answer is no. But the reason I asked the other questions is because the D500 is a VERY good camera. It will probably do whatever you are trying to accomplish. It can make large and beautiful prints.

There are many programs that can do 'up-rezzing' for printing larger, and many do a good job, but as pointed out above by others the process uses algorithms to create more pixels based on things like 'nearest neighbor pixels'
It is guessing, but not creating true detail. That is why no software can turn a 20MP crop camera into a 40mp FF camera.
I have made absolutely gorgeous prints from 8MP & 12MP cameras so it sounds to me like you are selling your camera short. Perhaps sharing the specific problem you are trying to solve would help.
Good Luck

While it is very interesting that Technology can do these things, and over time they will no doubt get better at creating these approximations of reality , they are still just that, digital approximations. 'A' waterfall with mountains and trees... but never 'THE' waterfall with mountains and trees that you saw at a particular time in very specific light.
'A' human like face, but never the face of someone you love as you saw them.
There is no specificity or context.
Film has been 'dead' for 20 years, yet lots of people who so desire are using it
I do not see it as a threat as much as the evolution of technology that we could not stop if we wanted to.
It is not without issues, the 'discriminator' component of the technology can only work because it was 'trained' using real photographs either purchased or taken without the creator's permission.
But creative people always find a way to be creative, they show us new things , and familiar things in new ways.
I don't see that changing much.
Photography's magic has always been specificity, the ability to capture and preserve not only the Thing (event, location, person, action,_ but also context of time and light, and preserve it.
No matter how good, approximations can never be the thing itself.
Don't worry, be Happy......

This was a 'Fortuitous Accident', It is important to understand the difference between RAW & JPEG.
There is nothing wrong with your shooting JPEG, if you would rather not be tied to post processing every RAW file.
You can also shoot RAW + JPEG and use JPEG output most of the time and only go to RAW files when more processing seems necessary to you.
While it is true that most experienced Photographers shoot RAW files for the most flexibility and the most image data, Modern Cameras produce very good JPEG results. A casual photographer is probably better off shooting JPEGs
Your 'Accident' showed the difference clearly.
Now the 'full disclosure part' You will notice that JPEG files are much smaller than RAW files. Jopeg files use what is called "Lossy Compression" meaning that once the file is compressed in camera (according to the settings you make) the file is compressed and all data not used for that 'LOOK' is mostly discarded. As much as 80%.
That sounds worse than it is. If you are happy with the look, and not having to edit, then all is good.
If you want to revisit and re edit the file, you cannot re-create the data that was thrown away. So editing is more limited.
If this is a hobby, and you would rather not edit, JPEGs are great. JPEGs are even required in some professional circumstances like Spot News, Reportage and some Sports.
As long as you understand the tradeoff, do what is easiest for you.

Well, I wish you luck, the fact that it was wet for a long time, battery in, makes it difficult dampness & Air over time causes corrosion. The advice above to get battery out & open everything that can be opened and perhaps a small fan - not too close.
Apple has recently come out with an advisory against using Rice for dunked phones & watches.
And would be Very careful about any heat --- perhaps a hair dryer at the lowest setting from 2 feet away just to lower the relative humidity (not to warm the camera)
I sincerely hope you get lucky, but would temper expectations as 5 days wet is a lot.
You might check if the battery will take a charge (out of the camera)

A DSLR with a very large lens , that extends and changes the center of gravity is a very challenging load for ANY tripod head. Most tripod head load capacities are optimistic , and at best indicative of static load capacity.
When you have a lens extended, it acts as a lever on the tripod head. This means you need a multiple of the camera and lens weight in 'capacity'
I am sure the head will hold your camera, but the question is will it be rigid enough for the real world on a windy day.
I would look at a beefier non geared head. (BTW it is unusual to use long tele's on geared heads) Geared heads are great for the studio, Architecture or anywhere slow precise positioning is most important. (That's why they are shown with smaller lenses)
I do not know the head you are talking about, Sunway is a Chinese company that makes lots of products, I own one piece of their gear, a leveling platform. Not bad not great. But the weight rating should set off alarm bells if you cannot try it in person.

I do not know how you intend to use that combination, but thought it would be worth thinking about.
Good Luck

The only true protection is proper backup, local, off site and or cloud
I always do 2 local so I never have to deal with restoring from the cloud, but I still do the cloud.
Take a look at OWC (OtherWorld Computing) I have never had one of their drives fail and their products are really nice.

I recommended Ruth Orkin's book above as a really nice one. We only know a few things about the person.....
She was injured hiking, and she will have reduced mobility for some period of time. Her new potential interest is Photography.
I started with Ruth Orkin because she is a great Photographer, who had a famous project , a world through my window. My Idea was inspiration first, and a book that would be worth having even if her interest in photography should wane. But certainly other books would be welcome and informative, and, since we have no idea what will appeal to her, we can let the giver decide. My sense has always been to light the fire with inspiration first then feed it with 'how to' later....because if someone has interest, they will be more inclined to figure stuff out..... which is a great way to learn.

The issue is battery quality more than brand name. There ARE aftermarket batteries that provide very good service, there are some that don't. The way Chinese manufacturing works is that they will make any quality level you pay for, in the Same factory.
If you want a premium product , they can make that, If you want to build down to a cheaper price, they will do that as well. Name brand Batteries buy you quality control. Some third party batteries work well.
If you get a new camera (to you) and need one or two extra batteries the cost of brand name is not too onerous.
If you have several cameras that are used extensively and need many batteries, the better third party options can save you money.
I have done both, and in my experience, I do get longer life from OEM batteries
Battery chargers with two slots & simultaneous charging ARE helpful if you are a heavy shooter.

If you can find a copy of "A World Through My Window" by Ruth Orkin, It would be a wonderful gift.
Ruth Orkin was a very gifted (and much Collected) Photographer, who had a side project of photographs from her window.
It would give her inspiration to start immediately