Another case that sounds like what happened to my niece's Pixel 4 XL. Rather than repeat myself yet again, I'll refer you to the answer itself.
[post|740760|Diagnose Pixel 4 XL not booting - Google Pixel 4 XL - iFixit|new_window=true]
The upshot is I couldn't find anything wrong, but when I plugged the battery back in after poking around the insides, it booted back up like nothing was ever wrong with it. So in her case the fix was physically unplugging the battery.
Unfortunately, I haven't gotten any feedback on whether that fix worked for anyone else yet but given everything else you've tried, it couldn't hurt.