If you already have modern cameras with "bells and whistles", what is your motivation for getting this?
Are the "bells and whistles" cameras too complicated, too large, too heavy??? Is this a "Gas Attack"?

The fact that you are considering prime lenses eliminates bridge cameras and pretty much locks you into either mirrorless or dslr. The fact that you indicate that you are not looking for a modern camera, eliminates mirrorless.
So, you are looking for an old series DSLR with either a kit zoom around 35-50mm or with 35 & 50mm primes.

I would suggest looking into older versions of your modern cameras that could use the lenses you already have.

Make sure you are in "Desktop" mode and not "Tablet" mode.
Also make sure Windows 10 is up to date

It really boils down to:

1-$$$ available
2-$$$ available and
3-Just how important is it to you to have the best?

I am going to assume you shoot nature/birding.
Do you plan to hand hold or mount?
Is IS, VC, OS really important if you are shooting at 1/700 to 1/1500 shutter speed?
Do you plan to shoot in the rain? Is your camera weather sealed?

As you can see, everyone is all over the place with their third party lens choices however most everyone agrees that the Canon L II is the best and the most expensive.

I choose the Sigma initially because with a recent, at the time, firmware upgrade the focus speed and OS improved significantly and it cost less than the Tamron. I tested both, in April 2018, and prefered the Sigma. Roughly a year later I had the opportunity to buy the Canon new at a reduced price and did so.
While the Tamron folks will tell you it is a "faster" lens wide open than the Sigma, it is only roughly 1/3 stop faster not a big deal. The Tamron does have weather sealing and costs a bit more.
There are tripod/monopod mounts available for both and neither come with one.

Good luck with your choice and enjoy!

Regarding the Sigma, it is lightweight. I always shoot it hand held.
It is, I believe, the lightest of the 3 and IMHO provides the second best images.
If the Canon Lens you are talking about is the version II, it is a bit heavier however is a much better lens than the other two.
Before I bought the Canon II, I researched and tried the Tamron and Sigma. I bought the Sigma.
It now stays in my closet as a back up to my Canon.

Which Canon Version, I or II?

I would suggest trying the SX60, zoomed out, tracking birds in flight before going on your trip.
If you haven't done it yet, you will find the exercise illuminating.

Ditto in Northern VA.
Looks like winter took the year off, although March has a tendency to bring some snow.
My bones prefer the warmer weather.

A lot of free time and patience.

For convenience sake, I now travel with a Bridge Camera in my suitcase.
Prior to that I would put my kit in a backpack and carry it on.
Never had any issues with TSA.
I travel a lot and go through Clear and TSA Pre-Check, I do not know the experience going through normal security lines.
One thing to remember, if you are going on a small jet you will probably be asked to gate check your carry on. Make sure to remove all batteries from your bag before gate checking it.

You don't mention which Canon or Sony Camera you use.

When I go out on a walk, I always shoot 1/1000 or faster, Auto ISO and Manual, thus eliminating the need to worrying about light changes or for a tripod or monopod.
Besides, there is always a tree or post to use for support.
On your Canon, make sure you set AF for AI Servo, High speed continuous and finally zone AF.
If I am on my own and am going to post in the same spot for a long time, I will bring a tripod and Gimbal Head.

I would suggest you rent a EF 100-400mm f/4.5-5.6L IS II USM lens and a 1.4x extender if your camera will auto focus at f/8. The lens is amazing and the extender will give you more reach. While the 70-300II is a very fast to focus lens, it doesn't quite have the reach and you can't use an extender with it.

Enjoy and post some pictures.

It is a "Great" lens.

Great Company, I have not had any issues dealing with them.

Considering he is concerned with BIF, he would be shooting at 400mm regardless and the lens is "Great" glass.
He has a 55-250mm, the 100-400 would be redundant for more than half its range. I don't see any used EF 100-400L II for under $1100.00.
Go prime "L" Glass for your nature photography.