Buying advice - swimming camera!

You're exploring questions that most of us have, and I'm not sure there is really an answer here.

As you go up in image quality, the tendency for the flexibility of the camera seems to go down.   You basically end up with very specialized equipment.  In my case I went from a series of Canon point-n-shoots to an RX100 II in a Nauticam housing with dual strobes and then to a D810 in a Nauticam housing.  Currently a D850 in a Nauticam housing.

With my Canons and the Sony, I could shoot a wide range of subjects, but not true macro, and not really very wide either.   But I had zooming at least.

With the RX100 purchase, I was hoping to solve the problems of the point-n-shoots.  First by adding dual strobes, which just made an absolutely huge difference.   Second, by fixing the slow autofocus issue of the point-n-shoots.  (Last one of those was a Canon SD870is by the way).

The reason I only used the RX100 for a year was that it did not solve the slow autofocus issue of the point-n-shoots.  Compared to the Canons I had been using, I essentially had a bigger-sensored point-n-shoot with a significantly worse user interface.  (Sony menus suck.)

At least I had more flexbility, supposedly.  I bought a pair of wet lenses, a small dome port for wide angle and a macro port.  I never ended up even trying the wide dome, and I couldn't get the macro lens to focus.  (I think I needed to be much much closer).

With the D810, my macro issues were solved.  My autofocus speed issues were solved.  My resolution issues (cropping) were solved.  With my strobes, I never had such high quality images, especially macro.   My wide angle issues were ... moved along.  I had bad corners before, and not much width.  Now I had extreme width, but corners were still poor even stopping down a lot.

What did I give up?  Flexibility.   First off, I had two lens choices: 16-35 and 105vr.   If you look around you'll see that FX cameras underwater have a very limited set of lens options, with nothing in the midrange.

Also, portability.  My camera gear now fills a carry-on roller bag, a carry-on waist bag AND a bit more space in a suitcase.  Carrying the camera to the boat reminds me that the rig weighs more than 30 pounds out of the water.

Compared to your current camera, you could get much better macro, at the cost of being able to shoot almost anything else on the same dive.   Put on a wide angle lens and you can forget macro on that dive.   Want to shoot a diver from head to fin?   If you have a 105mm lens on, you're backing up so far you can't use flash and you'll get a lot of backscatter and particles between you.     You definitely give up things when moving to an interchangeable lens camera.

But - those big-sensor point-n-shoots may or may not have enough autofocus speed for you.   I know the newer Sony's are faster than the older ones, but I don't know if they rival a DSLR yet, and DSLR focus speed is where you want to be.