In case anyone still has problems, I kept working on this....
Somehow my 32gb SanDisk memory card was corrupted or damaged during the upgrade. I tried tried transferring files off, reformatting, and putting files back on but the media issue still returned.
I ultimately just moved my files to an old 16gb Adata card I had around, and the background media problem vanished. No more battery drain or phone getting hot, and everything works as it should.
Reformatted the SanDisk card with SDformatter, and it appears fine for all other uses, best I can tell.
Soooo... battery drain was coming from some kind of SD card issue. There seem to be lots of complaints about the 32gb SanDisk cards working in android phones elsewhere....