So in the end, I went with an R6 III. 🤷♂️ How did I end up here, you may ask.
I originally wanted R7 and R8, but had a hard time reconciling their limitations (R7 = slow readout, crazy 4K60 crop, AF issues, shutter issues; R8 = no IBIS, tiny battery). So I was still stuck without a "hero" camera. Something I could use for handheld video in most situations, and for photo on the same job if needed. I decided to get an R6 II, due to the holiday discounts that I hadn't anticipated earlier, to get past all limitations. Though now, I'd need to get a Ninja V+ to get the highest quality out of it ($500ish used), but I was OK with that.
And then I got a deal I couldn't refuse on a new R6 III, which would end up costing the same as the R6 II/V+ combo, while exceeding even their limitations. Grabbed a 28-70/2.8 STM and Godox V480C while I was at it. Everything arrived at the very last second, and I was able to use the combo for the Xmas Eve job I had lined up that same night (nightclub photo/video).
Photo AF was about on par with my A9 in extreme low light (good but not amazing). Got hung up on too many unintended faces, so I turned off subject detection. Video AF was definitely better than photo (no noticeable wandering/searching), probably because it moves slower. Lighting was super dark, so I was at ISO 25,600 4K24 video/6400 photo + flash, but the video quality held up thanks to the second base ISO being close. Added Canon's own CMT LUT and some Topaz NR, and the video was good to go!
Vs my old OM-1, it's not even a contest. R6 III feels like a decade newer. Even the IBIS is MUCH smoother with more natural movement, despite everyone constantly blanket-praising m4/3 IBIS. It's hard to make it look bad if you move somewhat deliberately (slow walking through a crowd), while OM-1 would twitch at random, no matter how smooth you were, even with plain old panning. And OM-1 would randomly lose video focus in much brighter situations than that night, no matter how large the focus box was. R6 III stayed in place through some absolute hell situations (nightclub strobes), though photo AF completely choked at that point. Wondering if it was the 1/8 mist filter causing AF issues (no noticeable flaring), but it doesn't matter. I got all the shots I needed, the client was happy with the output, and the R6 III roughly worked as expected.