So, after all that... anticlimactic and boring news. After re-downloading and enabling every mod I had favorited for easy re-download, apparently I missed 10 mods, 'cause now I have 40. I've remembered 5, and re-downloaded 4 of them. I don't need the 5th anymore, so. The good(?) news is that none of those 40 seem to be the problem, and the 40 include Sim Settlements & RotC. The bad news is I have no idea what was causing it. It also can't be the save considering I didn't make a new one to test, just continued on with my save, and the crashing stopped once I deleted my load order. I have no idea what I'm missing, but things seem to be working now.
I'm doing one final thing, which is actually using SS & RotC again. Post-load order deleting, I downloaded and enabled them, but didn't bother with settlement stuff. I'm going to activate them now and see if the crashes start up again or not. I'll update sometime tomorrow, so far it's been a couple hours with them active and doing stuff and I've only crashed once, at the beginning. That could just be normal though, crashes every now and then are pretty normal for me.
(Also sorry if this is considered spam, wanted to collect all the quotes together so I could easily refer to this comment of mine to refresh my horrible memory.)