If you chose the Canon 100-400, perhaps my experience may be of help and it was also experienced by many, but not all Canon 100-400 owners.
In past experience, the older push pull Canon 100-400 was capable of outstanding images. I had one and it performed very. The down side was that some would go out of spec and lose the top level of image quality. My girlfriend had that happen. She returned the lens to Canon for re calibration and it was outstanding again. So, when purchasing be careful and test your lens. My suggestion is to consider either a vendor who will take a return in a reasonable time, or if buying on ebay or equivalent make sure the seller will accept a return. You can do outstanding work with the 133*400. Also a good Canon 400 f5.6 has a quality benefit over the zoom, but less versatility. Your choice. Enjoy the BIF challenges. Best of luck

Several years ago, I was at a seminar given by Nat Geo. Two Nat Geo photographers talked. Both used the Micro 4/3 system for some shoots. I am a friend of one and after some communications, settled on an Olympus E-M1. I then bought two bodies, 5 lenses ( with the 200% crop factor) and put them plus two flashes, 7 batteries, a charger, some filters, etc into a fanny pack. I went to Ethiopia for two weeks, trusting the recommendation. I have printed tack sharp images up to 30x45" ( some on aluminum, most on paper). Now the comparison. The 35mm equivalent range was 14 to 600mm. While I mostly used the consumer lenses, I had a super wide ( 7-14mm equivalent of 14-28mm )
My Canon system with equivalent lenses etc weighed nearly 45 pounds and required a large backpack. The Micro 4.3 system including the fanny pack ( well stuffed but manageable) weighed 10 1/2 pounds. I always had the two bodies and lenses available without having to take off a backpack and open it. It does not look like I an carrying blatantly expensive equipment in distant lands, inviting theft either.

Now that I have added the pro level lenses which weigh more, for some trips and uses, it weighs more and I have probably too many lenses and am sorting them out. I bought a 300mm f4 which is equivalent to a 600mm f4, amazing image stabilization and again very lightweight.
Since I wanted to be able to make larger prints of outstanding scenes so I also bought the Sony A7R II. I added a vertical grip with wireless remote and am delighted with that also, but take the Oly for most uses. The quality surprises, but with the small sensor, great low light performance is not as good as as the Sony.
While in full frame, there is little weight advantage, there is, with the Sony, greater dynamic range, and great low light useage. It is slightly smaller, but that is not the reason to change for me. The Oly or Panasonic Micro 4/3 systems are outstanding up to a point and it is for all of us to determine what that point is and what compromises you are willing to make for your needs. No system is perfect. I shot Nikon for 30 years, switched to Canon for 20 and how use mirrorless. All based on personal choices. My belief is that no system is the best, brand arguments are senseless since so many manufacturers make systems which are among the top used by professional and top rated amateurs. It comes down to what feels best, meets your economic choices and how each brand, with its individual pluses and minuses works for each of you. All have benefits and detriments and we need to see how things work for us, not what public opinion dictates, both in photography and our life choices.

All suggested will work. You might consider checking used old Linhofs, or any of a long list of used ball heads which will perform very well. While I have a RRS 55, I also have some cheaper and smaller heads on lighter tripods. All work if you work within their limits.

Olympus Tough. water and weather proof.

Just like asking opinions from lawyers. Get 10 lawyers if you want 15 opinions.
For better response, inform us of what your goals are and what you usually shoot., It makes a lot of difference if you do wildlife and birds, rather than if you are primarily a landscape photographer.
A more informative question would create more focused ( pun intended) answer.

Please define your end result goal. If you want to print small, less than 8x10, or do slide shows or online sharing, the end goal is much different than if you want to make large prints, or expect to do major crops, submit to critical publications, etc.

Which part of Antelope Canyon did you visit, upper or lower?

The issue of too small for your hands, was solved for me with the newer and larger versions of m 4/3, for me Oly Om-D E-M1 and E-M1 MK II. They fit me fine. Try before you Buy to see which works best for you.. What you use for your work is personal choice based on familiarity and useage. What works best for you is great, but does not always translate the same way for the next person.
We already have cameras which can create amazing images if used correctly. The next generations are going to have more bells and whistles. Some of us will buy and chase technology and some will not. There are still people shooting large format film cameras. Some are going to the high quality DSLR med format cameras and very pleased and then there are the majority who use their cell phones successfully. It is the old saying, "what floats your boat" that works for most of us.

having successfully printed 30x45 with Olympus E-M1 M 4/3 camera and quality lenses, I loved the weight reduction especially on long trips and remote areas. Now, in addition to the m 4/3, I amusing the Sony A7R Mk II with ability to print larger, or make quality crops. The mirrorless vs DSLR is personal choice as to which works for you. Both come in large and small sensor versions. So, evaluate before buying and then learn your new system well.

Find a good camera shop. The camera brand means less than what fits you and your budget. Some recommendations have been made here with no idea of your finances, total interest, etc. There are so many variables and also besides the DSLR there are mirrorless cameras which are taking a large share of the camera sales, so that is another item to consider.
Decide what your goals are before buying anything or believing the claims that this or that camera is best since there is no overall best. IF possible, join a camera club, see what is accomplished with different cameras. If you end goals are to sell very large prints ( 4x6 feet or more) it would require different equipment than if you just want to share photos online, or make small prints for the family.
Education is the most important issue at this time. Unless you have a large disposable pool of cash, this is the best way to handle things. So your personal goals, finances, etc make a huge difference in what your choices should be. I have shot large format up to 8x10", through very small sensor cameras and what is called full frame. Each was a learning experience in choosing what I use now. Good luck, educate first then chose. IT is like the carpenters advice, measure twice or three times, then cut once.

We reward people for not working and call it welfare. Able bodied people grow up in this type of society and realize that in their culture it is how they were trained. It may be politically incorrect to ask them to work for the hand out, but they also vote. Democracies fail when those who vote for a living outnumber those who work for a living. While there are those who are truely in need, there are too many who can work and chose not to. IF the government gives money, it should require those who are able to work for that money.

I have had tack sharp prints on aluminum from a 16 mb file at 30x45". A quality lab and good selection and processing of the image to use helps. Larger files can be easier and have greater success also. Going to a full frame from a cropped sensor can be a benefit, or of no real use. IT depends on the skill of the photographer in pre and post operations.

Gradual ND filters may be what the question is about. They are used when part of an image is much brighter to bring the dynamic range within the camera's recording ability. A full ND filter us usually used to slow the shutter speed for longer exposures for example to make water looks silky, whereas a grad ND filter would be used for darkening a sky or highlight which is beyond the range of the camera to capture while capturing the rest of the image. Exposure bracketing can work, ND grads can work. It depends on your choice and experience level to chose.

Unless special needs, what you have is excellent. Make a list of what you are missing. Upgrading just because a company put out a new camera is just paying that brand. Do you need greater dynamic range lighter camera with more pixels? What is your end result, larger prints, more ability to crop smaller portions of the image, or just keeping up with the others who like to buy new, bragging rights? If what you have meets your needs and desires, keep it until it fails and repairs are more than its value. Otherwise, make a list of benefits and detriments of each model. The ultimate decision is not ours to make, just compare what you get with what you have and what you might get if you change.
Good luck.

In regard to the Black Rapid and its clones attachment style. I too found their attachment system annoying. An easy solution for me since all equipment which can has quick release attachments, I just bought a quick release clamp and attached it to both my single and double rapid straps. Problem solved, faster and more utility than the screw in attachments.