Well, well... getting more specific on what the issue is (though still not sure about the root cause): I noticed that in some saves with Sorcerers, the issue didn't occur at all. At first, I thought maybe it starts occurring when the sims have custom needs. But nope, one of the saves, they had the custom Sorcerer needs and no issue.
Then I noticed where it started happening was a save where I had taken Sorcerers with custom needs into CAS with testingcheats on and cas.fulleditmode to add another sim.
So in one of my "fixed" saves (where the issue shouldn't be happening anymore), I tried making a couple Sorcerers again, making sure they had custom needs, and then taking them into CAS and adding a new sim to the household. I tried to "save" out of CAS and it hit me with the infinite loading screen and lastexception error.
Which means it's at least narrowed to having something to do with CAS, presumably, and possibly something to do with editing in CAS via cas.fulleditmode. Which could be related to the "fix" for not being able to edit townies in some ancillary way, though I'm not sure how. I'm guessing it also has something to do with editing in CAS specifically tied to custom needs, though I haven't fully tested to confirm that possibility.
Edit: This feels very Sisyphean, tracking down.
I've now found that new saves, seems no problem.
But whatever issue with old saves, does cling in some way. With the main Sorc family I was using for testing (I'll call them "The Bobsons" for simplicity) their save files start having the issue with when: Some of them had custom needs and I'd taken them into CAS with testingcheats on / cas.fulleditmode and added another sim to the family, saving it that way. (The issue did not appear until post-patch.)
I "cleaned" one of their bugged saves today by removing the mod and then making a copy of the save where they aren't Sorcerers. Upon doing some more testing just now, the infinite loading screen issue returned when I made them Sorcerers again to the point of them having custom needs and saved another copy, with them as Sorcerers (in that copy, the issue returned). When I did this with only making them Sorcerers, but NOT giving them the custom needs... the issue hasn't returned yet.
So whatever "corruption" happened connected to the CAS thing is clinging to the save and custom needs definitely appear to be tied to it in some way. This would explain why it seems to be a bizarrely fringe issue, since very few mods change the needs in any way.
