@Retrospectre
So I've made some progress, thanks to previous work from
@unknowndatax, and a lot of hand holding from spiritovod on rin. The saves use the same encryption keys as the switch version (so C5VbD9SJxe4FhK7wnWxy_LVSuHfbQjAU for the full version, D4FDrH92HO8ztZ1Bs492F4a3o9c3gixt for the demo version). There are a bunch of small changes made in the PC version, and so far only by changing everything to how it is on PC (i.e. ending up with the exact same file in the end) was I able to get a switch save to ""work"" on PC. If anyone has one, it would be super helpful to get a PC save with a decent amount of game progress in order to compare how things added to the file as the game goes on are handled (unfortunately we won't be able to get near like for like saves for that like we can for the initial save).
Attached is a collection of what I used and learned from my research, including a variety of saves from different platforms in both decrypted and encrypted forms (see files.txt), a disorganized text file with my findings/guesses about the differences between the files, a BMS script made by spiritovod for decrypting/encrypting saves (file needs to be edited to change between decryption and encryption or full game saves and demo saves - steps are in the files comments), and quickbms for using that script. While he's not invested enough to actively work to get the saves working, spiritovod has also offered to update the script for changes that can be easily automated, as well as give insight on the changes between save files, so if anyone has any specific questions (e.g. a value in the saves that you can't figure out the function of) I can contact him.
Hopefully someone with more experience in this kind of stuff can help further progress things.