Thank you for merging the posts and for the info about that mod. I got it and here is the info I've found:
In the file that no longer saves and is 222,000k in size, it says I'm using 64/150 Sims and there are 577 total in the game.
I re-opened a legacy family file I'd last played on July 21 that is only 18,000k. That one says I'm using 36/150 Sims and there are 607 total in the game.
In the "bad" file, I tried deleting all the Cupid's Corner Sims that I'd added and was playing with and brought my Sims total down from 64/150 to 20/150, but that didn't make a huge dent in the file size. It only went down about 200k.
What I did notice, though, reopening the bad file over and over is that it seems to be saving Build/Buy changes (like adding an object to the lot), but it's not saving when I sell any items from the family inventory, and it's not saving any of my Sims' progress going from day to day. It keeps going back to the same day, my Sim no longer has her job (which was the first thing I noticed missing when this all started happening Sept 2), but items she fished during the unsaved time are still in her inventory, yet her Fishing skill is back to 0 as if she's never fished at all. So it's almost like it's saving a few things, but not everything.
I do use OneDrive, but I've been using OneDrive since January (new computer got for Christmas) and haven't had any problems at all playing Sims almost daily until Sept 2 when my Lovestruck file suddenly wouldn't save anymore.
I have about 50 different saved games (I know! LOL! I start a lot of challenges.) and had been playing with them fine with no issues, so I'm not sure OneDrive is causing this. The problem file was started a couple days before Lovestruck came out and I've been playing that file exclusively since then.
The last good save of the Lovestruck file was on Aug 29. Unfortunately, I accidentally overwrote that one on Sept 2 while trying to figure out why things weren't saving. So the next "good" file I have for this same group of Sims is from August 3. I opened it last night and saved it as a new file. It was 75,000k in size on Aug 3. When I re-saved it and played it last night and saved again, it went up to 135,000k after I added a reno of Streamlet Single to Willow Creek. Do lots increase the file size that much? Seems high.
Maybe I'll have to go do some experimenting with deleting some lots I added and see what that does to the file size. I don't have all the lots in the game filled, but I have a lot of them occupied with something , but my 18,000k file from July 21 is the same way; I've built up most of the cities so my Sims have places to go (minus Cuidad since that wasn't there when I saved that file Jul 21.) and that file size hasn't blown up much.
I have managed to save the families I was playing with from the bad file to the tray and added them to a brand new game file I started Sep 3, and so far it's continuing to save okay. And the file is still small, only 8000k, but I'm still nervous about it happening again since I don't know what caused it. 😓
ETA: fix typos. I called OneDrive Overdrive. Oops!
Also, did some more tests with other families in the "bad" file and the build/buy stuff is mostly saving (a cat wand and a child's bike that were in personal inventory did not save), but the Sims progress is not saving at all. I removed some lots I'd downloaded, one was a 64x64, but the file size only went down to 119,000k.