Discovered part way into diagnosing that the battery is swollen and not holding any charge, so basically all diagnostics have taken place so far with power adapter only. Initially (when the battery was still connected) the MBP allowed me to boot from external disk into Catalina recovery, and First Aid showed no problems with the ssd. It allowed me to initiate Catalina installation onto a new volume, but it was extremely slow and the installation never completed and eventually rebooted into the loop. At this point I discovered battery and disconnected. At this point it does reboot loop even after selecting an installation file on external drive. Tested with known good ssd just for fun and same results. MBP does the reboot loop when trying to enter safe mode, the built-in recovery partition, and while using external display. Successfully allowed diagnostic startup and showed no problems other than no battery. Fans fine, doesn't feel too hot. SMC bypass mode successfully resulted in fans on max, and then the MBP turned itself off after a minute. I haven't begun testing the logic board itself yet, was hoping you might give me an idea where to start?