Hmm. I'll throw in my two cents, but I've not shot the new mirrorless cameras, nor do I feel any particular desire to. I've been shooting Nikon DSLR since 2004, and Nikon SLR since 1992. My mirrorless experience has all been with point-n-shoot cameras, mostly Canon, and then a Sony RX100 II. I've shot all of those mirrorless cameras underwater, and the Nikon D810 and D850 also.
Initially I was drawn to mirrorless (point-n-shoots) for IQ and price. IQ meaning 'better than the ISO400 disposable film camera' I used first. But I was drawn to the live view experience as well, being able to see my composition in the rear LCD while swimming along or just holding the camera out with one hand.
Ultimately I was tired of missing the shots I so often missed with point-n-shoots, due to poor autofocus. This colors my perceptions somewhat as I became more and more demanding of a camera that quickly and precisely focused where I wanted it. Eye of a critter peeking out from behind something - wide point-n-shoot autofocus couldn't capture that. Fish swimming by - seems to get a lot of shots of fish tails swimming out of frame. I also got tired of the lack of colors, so I took the next step up: went to a 1" sensor (RX100) and added flash (dual YS-D1 strobes). The total cost of camera and rig (included a couple of wet lenses for macro and wide angle I never ended up using) was $5000. Nauticam housing. What I got was a nicer point-n-shoot that had huge advances over my previous cameras only because of adding flash. Autofocus was still too slow.
So MY choice was to get one of my DSLR's underwater so that I could finally resolve my autofocus issues. I bought a D810 Nauticam housing, a 105mm macro port and a 230mm dome port.
Here's where I can give some advice.
I really wanted much better autofocus with the DSLR, and I got it. I also added an expensive high-eyepoint eyepiece so I could more easily see everything from behind my mask. I spent a lot of money for this capability, and for macro I pretty much got what I wanted.
For wide angle, not so much, until recently. The recent addition was a Sea and Sea internal correction lens that really, really, really works well and fixes my corner issues with the 16-35 in the dome port. Should you end up with the Canon 16-35 in a full frame camera behind a big dome, you would be well-advised to get this correction lens too.
OK - so here's what I gave up, and you should be aware of these tradeoffs too, if not already from your current rig.
1. video. Yes, I can do video with the D810 or D850, but to be honest, mirrorless cameras have always been easier to shoot and tend to have better video autofocus. With any of my point-n-shoots I felt right at home switching to video mid-dive and filming a video sequence. The autofocus on the D850 during video is almost unusable, especially if the camera is also moving. In general, it's much more limiting.
2. focal range. All my mirrorless cameras had a mildly-wide to moderate telephoto zoom lens. To do wide angle, I needed a wet lens. To do macro, I needed a wet lens too. My DSLR choices are the opposite - no choices in the middle. I'm either wide to very wide (16-35) or I'm macro with the 105vr. The Canon choice would be the 90mm macro, and I think that's too short for full-frame.
3. travel-friendliness. I bet your current rig fits in a carry-on, or at least a suitcase. Mine doesn't. I use all of one roller bag for the housing, dome port, strobes and a few odds and ends. I put the camera in the housing, with a lens (or port) attached. I put the lenses and backup camera (RX100, in its small housing) in a waist bag, and then I still have the macro port and 90mm dome port extension (filled with clamps) in a suitcase. If you go full-frame, you go BIG. You also tend to have less lens choices.
I'm very pleased with the quality of the shots I get now, but the cost in $, size and weight is up there. A compromise well worth looking at is a DX/APS-C format DSLR. In the Nikon world that would a D500.
With a DX/APS-C format camera (mirrorless or not) you get noticeably smaller wide angle options. You also get more focal range options. You still have autofocus during video issues (with Nikon, not sure about Canon here).
If you value video a lot, lean toward mirrorless. If you value stills, lean toward a DSLR. That's primarily on the basis of better autofocus performance. Consider also that systems designed for mirrorless (including the lenses) may focus silently, while older design DSLR lenses can have a lot of audible clicking and whirring while focusing.