Circling back to answer my own question.
I ended up buying the new Sony ZV-E10 mirrorless, which is basically a more compact a6400 (so really small) with some slightly updated color processing, plus some better I/O that I've been using for livestreaming and also Zoom music practice.
The photo and video quality are a large and obvious step up from my Iphone X Max, both in low light and in bright light. I haven't even tried shooting/filming in raw yet---the difference is big with JPEG photos and default video (no LUT or anything). Haven't gotten to compare it to an Iphone 13 pro yet, but I love the thing so far.
It has builtin stereo mics, which the reviewers loved, but which in reality suck (harsh/cheap sounding) compared to the iphone X Max's builtin mics. But, they've been great to get a 'reference track' so I can waveform match a separate recording taken with real mics to the videos I take with the ZV-E10.