As per comments, courtesy Matthew, this seems to be the best option:
I think your brainstorm is the best option. You should get them all
child themes and then going forward it will be way less of a hassle to
update. It shouldn’t take too long. Just figure out which files were
customized, through them in a child theme, back up the customized
parent theme just in case, and then update the parent theme. Then it
should be way better. Not only will you need to do a long manual theme
update but in the future you can just update instead of transferring
customizations.