Hello, sorry for such delay. To my confusion, the problem seems to be resolved. I wish I could say what exactly did it. I think it must have had something to do with restarting my computer, which I hadn't been doing between each stage before. The first two times the issue was persistent, but after the fourth or fifth time, it started to run a little smoother, and then save regularly. I didn't want to say it was fixed too soon, because the last time I did it started messing up again, but it seems fine, or at least workable now. I do notice that around every fourth save per session, it still gets stuck, but honestly even that feels like a breath of fresh air after what I've been working with. Sorry for making a fuss over something that could've been solved with basic computer knowledge.
I'll still put my path here, just in case there's something that can be done to stop the game from freezing every third or fourth save, but if not, I'm glad it's at least going a bit easier. Thank you!
C:\Users\lenovo\Documents\Electronic Arts\The Sims 4\saves\Slot_00000002.save
Edit: to answer the other question, at first I could save other households in the same load file, then after a few days I couldn't save others either, which is still the case.