I've been shooting m43 for a half dozen years or so - O-MD EM-1 in Nauticam - and was expecting to upgrade to some new Olympus hotness before my next bit underwater trip. However, after a recent trip to the Cenotes, I decided that I need much better low-light capabilities, and the EM-5 Mk III wasn't going to give that to me. So I'm now thinking about full frame, and would love to get folks thoughts about the latest FF mirrorless options (especially A7R Mk IV and Nikon Z7), or whether I should stick with the tried and true SLR (more likely Canon for me from past warm experience with EOS before I switched to m43). I'm drawn to mirrorless because of the power of the EVF. I also like that the top end remain lighter & smaller than top end SLRs (more for daytime use than underwater). Finally, specifically with A7R Mk IV, the 61 MP and APS-C "mode" (or just cropping in post) means I can have similar # of pixels as I have now with my m43 camera, enjoying some of the weight & size benefits of the m43 system. Not all mind you, but damn is big glass heavy!
High ISO performance/noise levels seem to be fairly close among the top FF mirrorless/SLRs, with the nod still going to the SLRs I think. Focusing speed doesn't seem to be an issue with the latest crop of FF mirrorless at good light levels, but I found it is an issue at the unreasonably low levels that I tried quickly in the store today (ISO 10,000 and 1/15th of a second at f2.8 - whatever that translates to in LUX/foot candles). What I haven't yet discerned is how well/poorly top FF cameras would fare in something like the Cenotes (where in any case I'm going wide). Or if it is a consideration in macro (I'd presume a focusing light, but perhaps that's a bad guess?).
So! I'd love the thoughts of folks who have shot with the A7R Mk IV and Z7, as well as modern SLRs. What are the drawbacks of the latest crop of mirrorless cameras? Is SLR still unquestionably the way to go?