I used an Olympus M43 system that for several years and enjoyed it and got some really nice images with it, but sometimes felt frustrated by slightly slow focusing speed in poor light and a limited ability to crop without excessive image degradation. Although I could fit almost everything into a ThinkTank roller (I have always carried camera, lenses and a few other bits in a carry-on shoulder bag), I was encountering situations in which I could not count on getting the roller in the cabin, so I bought a Pelican case that could be safely checked if necessary. Once I came to terms with the idea of checking my gear, I began to feel that a larger system would not be much more trouble to transport especially after I added a WWL to the M43 stuff.
I did a lot of research and spoke to a lot of people. I was about to go with Nikon full frame but everything is larger for full frame, especially dome ports. With M43 and APS-C, I can use a 100mm or 140 dome and only occasionally take a 180 dome. That would be impossible with full frame. A 230mm dome is pretty much required for full frame WA. I am unwilling to deal with the logistics of traveling with that. I looked very seriously at the WACP, but a 4 kg lens has its own transport issues, and I am uncomfortable putting a heavy, multi-element, $5000 lens into luggage. I also added up the weight of the camera, housing, lens, strobes, arms and WACP and at something like 11 kg, realized carrying that thing to and from the boat each day would be unpleasant and having boat crews hand down the rig and take it from me at the end of the dive would make me rather unpopular with the crew.
So I went with a D500. For me, APS-C is the sweet spot. The format seems to be dying out, but I hope my D500 bodies will last a long time, and hopefully someone will continue to offer crop sensor cameras.
I kept most of my M43 stuff, thinking I would use it sometimes when I needed a smaller kit for travel, but it just sits, and I think it is time to sell it off.