@puzzlezaddict I think, tentatively anyway, that the problem is solved. I've spent the last four hours since arriving home from work laboring over this thing. I had to make OneDrive start syncing something before I could even find the icon (which was in the hidden-icons tray), but once I did, I paused syncing and took out the Sims 3 folder. Then I ran the game through the EA app so that it generated a clean folder, and it finally worked without crashing. I put it through several tests: first, with no existing saves placed into the new folder, and using an EA world; second, with no existing saves and using a player-created world; third, with my most recently created save. The game worked fine on all three tests.
I spent probably an hour digging around my OneDrive files in search of four saves that I had accidentally trashed when I uninstalled and reinstalled the entire game last week, in my first attempt to fix this whole problem. Luckily I managed to find them, restore them and redownload them; but that's the only good thing OneDrive has done for me. Once I got those saves back, I followed the instructions you linked to deactivate OneDrive. I've found it more of a headache than anything else and wouldn't mind uninstalling it; I probably will if I start seeing a lot of messages asking me if I want to back everything up to OneDrive. I use this computer solely to play The Sims 3; so other than those lost saves, I've never had anything I really cared about backed up to OneDrive and wouldn't miss it.
After finding that the game runs fine now, I'm guessing there must have been some kind of corrupted file somewhere in the old game folder. Months ago I set up separate backup folders for downloaded CC, store content, and mods, so I have clean copies of everything I use in the game. I figure I'll have to manually reinstall all of it from those clean copies so that I don't end up with another corrupted folder; and at the same time it'll give me a chance to cull out CC I don't use anymore. If there's an alternative to that, I would be very glad to find out about it.
So I think the problem has been solved, but if this is a fluke and it decides to stop working for me later, I'll post here. For now, many thanks for all your help -- I'm so grateful for your willingness to spend time and effort on my problem!
EDIT ... I installed a recently created save and started playing, only to find that I was playing with nothing but the base game. I went back to the launcher and checked all the EPs and SPs; then I tried to install some CC, and all I got was a message saying there were no packages to install. I got the same problem with one store-content item, even after I redownloaded it from the Sims 3 store site. I've been researching this issue, especially since when I got the EA app and it ran the game successfully, I noticed I'd been booted up to patch 1.69 where I had 1.67 before (and no issues with that regarding CC and store stuff). I saw a post from someone else who had this exact same problem, and it contained a link to a thread with a possible solution, so I'll give that a try first and see what happens.