Interesting debugging! Thank you for giving so much effort to go through this.
This reminds me of a very similar issue reported for v1.0 Pisound boards, but it happens exclusively with Pi 4 versions: PiSound with Raspberry Pi 4, because its power supply design got changed. The Pi 3 and earlier models didn’t have this issue.
To verify that it’s indeed SD card overheating, you may try to keep Pisound off, and heat the sd card area using some hair dryer or heat gun (don’t get it too hot though!), and see if you can reproduce the boot issue just by heating the SD card area.