My current setup is the Olympus EM10 in the AOI housing, with the 60mm lens and a single MF-2 flash. I'm really happy with the results with smaller subjects and am sure I have a long way to go getting better with just this configuration. I also really appreciate how small the rig is - very casual to dive with and easy for travel.

But I find I'm struggling with larger subjects - reef squid, octopus, seahorses, even bigger nudibranchs. It's hard to get the whole thing in frame, and at the distance I need to be I often don't have enough light. Definitely true for turtles, rays etc.

I'm curious if there's a setup that can manage fish portrait of even some bigger species through to macro on the same dive. I'd definitely be fine with it not covering wide angle. I'd be happy to consider changing formats/camera bodies too. I only really view pictures digitally so don't need super high megapixels in case a compact is one of the good options.

I was considering something like the 12-40 zoom with a wet lens if anyone has tried it, but curious to hear recommendations of any kind!

The newly released Nauticam MFO-3 does just this, it increases the field of view of macro lenses. In your case it would be the equivalent of about a 35mm macro lens. It is not available just yet, but due very soon. Attaches either with a bayonet of flip adapter or M67 thread. I've ordered one as I have the same issue as you with seahorses, larger fish, weedy sea dragons etc. You would need to check compatibility with your AOI port.

Your other alternative might be either a 12-40 or 12-45 lens in a small dome. The 12-40 achieves 0.3x at max zoom, which is about 55-60mm across the frame, but it gets significantly wider field than the MFO-3. These lenses won't work well with a wet lens as they extend when they zoom.

A third alternative is the WWL type setup using one of the 14-42 lenses, but this doesn't get as much magnification as you can get with 12-40 lens