12 hours ago, Chris Ross said:

The size and weight penalty for macro is certainly less than it is for wide angle, but you might stretch the definition to include WA macro which can be done with m43 with a lot smaller gear. I certainly find framing up easier with a macro lens without diopter you can find the subject from a distance and slowly move in closer and re-focus as you go.

So for me I don't think I'd change from what I have now. An OM-1 with 60mm macro, though I might consider adding the 90mm macro at some point. It's not a video machine but it will shoot no frills video. The choice may change depending on how much interest there is for video and what you want to do with it.

I disagree concerning the size and weight penalty for macro vs. WA:

=> When I use A7R5/Nauticam housing with Sony 90mm plus flip holder for SMC-1, this makes the biggest and heaviest setup I can have with this camera. Maybe comparable to WACP-C/28-60mm on the WA side (Canon 8-15mm with 140mm dome is certainly smaller and lighter)...

This macro setup gives similar AOVs and magnification as I had before with the ridiculously small EM1II plus bare Pana 45mm. I would say with macro the difference is the most pronounced between MFT and FF, at least as I am using these cameras (For WA I was using mostly Canon 8-15mm with 140mm domeport with EM1II, what gives pretty similar size between the EM1II and A7R5 setups)).

=> With FF, however, I get 61 Mpixel file with 14-bit for postprocessing...

"unlimited Budget?"

Easy......

I would hire someone a lot better than I am to dive with me, And also pay their expenses. Have them agree the images/videos and copyright are mine. We swim around and I point to what I want photographed/filmed and then claim the image/video later. Problem solved. 😉

If video is your primary use, I would suggest that a monitor is likely to be more useful than an enhanced viewfinder. It is a lot more surface area to push through the water, but it makes it so much easier to compose your shot especially with moving subjects, that it is worth the effort - in my opinion anyway.

11 hours ago, Architeuthis said:

I disagree concerning the size and weight penalty for macro vs. WA:

=> When I use A7R5/Nauticam housing with Sony 90mm plus flip holder for SMC-1, this makes the biggest and heaviest setup I can have with this camera. Maybe comparable to WACP-C/28-60mm on the WA side (Canon 8-15mm with 140mm dome is certainly smaller and lighter)...

This macro setup gives similar AOVs and magnification as I had before with the ridiculously small EM1II plus bare Pana 45mm. I would say with macro the difference is the most pronounced between MFT and FF, at least as I am using these cameras (For WA I was using mostly Canon 8-15mm with 140mm domeport with EM1II, what gives pretty similar size between the EM1II and A7R5 setups)).

=> With FF, however, I get 61 Mpixel file with 14-bit for postprocessing...

This is true, though you can certainly have some very light WA setups in m43 like a 4" zen dome with 8mm fisheye and a 230mm dome never enters imto consideration.

18 hours ago, Architeuthis said:

One consideration is the sensor: FF with low S/N and 14-bit/pixel and high Mpixel count vs. MFT with 12-bit/pixel and lower Mpixel count (and APS-C in the middle, but with 14-bit/pixel). I wonder how relevant this is for (mostly) flash enlighted and uncropped macro photos (maybe gigantic printouts are an exception)...

The wide dynamic range will rarely come into play in macro shooting with artificial light. UW contrast tends to be quite low much of the time, one exception being sunballs. MP is a similar story, mainly come into play when printing Large, natively a 20MP m43 sensor will give you 43 x 33 cm natively at 300 dpi which is close to A3 size. A well exposed image can be re-sampled to around double that resolution for printing with good results. The main benefit of big dynamic range is preventing posterisation in gradients like a slow gradient of surface water, you need to process in 16 bit to get this benefit. I think 12 bit vs 14 bit is likely little difference for this purpose.

I think for most people the benefits are small to not there for 99% of their images. It's more case of if you can afford and want to and are prepared for the limitations sure go for full frame. If you are making your living from it - I expect there are benefits for those cases where the extra quality is required. But there's plenty of people taking great shots with m43 professionally as well. This guy takes some amazing shots with an OM-1: https://500px.com/p/sulasulacom?view=photos - no UW stuff but some of the high ISO bird images are quite amazing!

When macro is your main focus and video is also important I tend to agree that FF is not the way to go despite the "infinity budget". The best underwater macro videos in the recent years are produced, in my opinion (which is shared by many juries of international competitions), by Alfred Schaschl. You can see his work here:

https://www.youtube.com/@alfredsch.5440

Fredi uses Panasonic GH5 II camera, so a 4/3 crop sensor. I found APS-C is a good compromise for both macro and wide angle, and I'm locked into the Sony system, so if I would have a big budget I would probably upgrade my A6400 to FX30 and not FX3. Unfortunately the 50-60 mm macro range is not well covered with modern macro lenses in Sony environment, but for video, I think the available lenses are OK. Fast AF for macro video is not really needed. You will film on a tripod anyway (macro), and when my subject is in focus I usually switch to MF, just to avoid the the AF is distracted by something else in the background.