Apologies for the delayed response!
Just confirming for others but yes, you're 100% correct - save files can be found in AppData/LocalLow/Devon Wiersma/Paintable Saves and can be zipped/copy-pasted out to other folks or backed up from there.
Officially supporting save sharing actually isn't something I had considered when originally developing the game, but it's definitely an interesting idea. Honestly the save system has been my white whale throughout development and even just optimizing them for sharing is something I'd be interested in tackling. I'll add revisiting the save system to my list and see if I'd be able to spin up something that could make it easier, who knows! :)
Also, if you and your friends make any cool art please do feel free to share as well - this idea sounds very fun and I'd love to see what y'all make!