I am in a similar situation, but have no hurry (but I think at the latest in 2023 I will upgrade and try out FF by myself). I am currently with MFT (Oly EM1II & EM5II), the difference to Massimo is that I am not very interested in video - I make them very occasionally and any format suffices for me. There are several points I am considering, that I list below:
#1.: What do I expect from upgrading from FF to MFT?
Do be honest, after reading many dozends of reviews and articles on different sensor size, over the water and UW, I still do not know. I hope to get improvements in:
1. a.: ultimate image sharpness (especially for macro), brought by bigger sensor and more pixels..
1.b.: more DR for WA.
1.c.: 14-bit together with 1.b. should give more potential for postprocessing in LR.
#2.: Since I will buy into a new system, all brands are an option to me:
2.a.: Nikon. Certainly not. Only the oversized Z9 seems to have comparable AF to the other brands. Even more important, old lenses, important for UW do not work with the mirrorless bodies. Adapters for Sony E-mount and Canon EF-mount lenses exist, but why buy into a system that con only be used with third party adapters and second party lenses? Also the dieing out and not future proof DSLR line is not an option for me, since I am starting from scratch...
2.b.: Sony is certainly the best option, when only the camera housing itself is considered: small, even with Nauticam housing and best sensor technology, regarding IQ.
A1: very high IQ and fast AF.
A7Rv: best IQ and AI AF module. I am not sure, however, whether the AI AF module, with automatic recognition of multiple objects, will be superior to older models for the kind of AF, how I perform it: single point dumb AF in the center and then track the point while framing (C-AF&track).
A7Riv: best IQ and maybe same AF performance in practice for UW, compared to A7Rv (and maybe also not much worse than A1).
=> Size and IQ vote for Sony, but the big drawback is lens choice. A single longer focal length macro (90 mm) and no native fisheye. So far this was not a problem, since the Canon EF lenses can be used with Metabones or Sigma adapter, but in the future, the new RF lenses will not work with Sony.
2.c.: Canon. Maybe sensor technology is a little bit behind Sony, but I think the difference is more on the academical level and not really relevant for practice. What stands against Canon (R5) is the sheer size of the Nauticam housing. A very big plus is the lens choices: all old EF lenses are reported to work perfect with adapter and it is not the question whether Canon will deliver a Zoom fisheye, but only when. It may be very well that they consider the demands of UW photographers and even come out with a 8mm - 28mm RF fisheye (no question, I would buy into the Canon system then). Also the new APS-C cameras (R7) look attractive for UW and maybe we (me and my wife) could upgrade the second next time to a R7 like body, instead continuing to use and upgrade MFT (e.g. to OM-1).
=> Size is a big problem with FF CAnon, but I cannot see Sony coming out with lenses that can compete with Canon UW.
#3.: WA is indeed a problem with FF, unlike MFT or APS-C.
I think I will start (and mostly use) the Canon 8mm - 15mm with Nauticam 140 domeport. I just will loose the (very practical) ability to zoom in occasionally (maybe I will use the Kenko 1.4x TC that I already have; On MFT I did not find a loss in center sharpness, when using it). Absolutely no idea what to take for regular WA. This May I joined Adam Hanlon and Matt (Yellowtang) on the Red Sea Odyssey and had the chance to hold a WAPC1 in my hands: I doubt that this monstrum is easier to transport than a 230mm domeport, the weight at least is twice, compared to a 230mm domeport...
#4.. The money: Well, did you know that the last skirt does not have a pocket, so that you could take money with you to the next level (not that I am rich, but I set priorities)
?
I am eager to read what others think about this interesting issue...
Wolfgang