Okay, since I had trouble finding answers to this question, I'm going to go ahead and answer myself with what I did, so that the next person with this problem at least has a little something to go on. I started with a fresh reinstallation of FO4. Then (and I'm leaving out all the fails here, btw) I started a new character, and got it just far enough that I could save. Then, I found where the game placed that save, and I copied my old saves there. (FO4 put them on my boot disk instead of on the same drive as FO4, even though there's also a User folder on the FO4 drive. So, Boot Disk C:\Users\your-user-name\Documents\My Games\Fallout4\Saves is still the first place to look, even if you install FO4 on a different drive.) That done, I did a fresh installation of NMM. After telling it where my FO4 was, I went with all the default directory options. (I did NOT let it reinstall my mods for me, since it kept trying to install old versions and stuff that I'd uninstalled to make room for mods I wanted more.) I then pulled up a copy of my old Plugins.txt file, and reinstalled what I actually wanted using that as a guide. Since my mod list is right up at the limit, this was (and still is, since I found a few new mods I want more) a long process--a few days so far, since I'm having to re-evaluate my whole list, but if you don't have too many mods, and know for sure which ones you want, it might only take a few minutes. As I worked on the NMM stuff, every time I reinstalled a half dozen or so mods, I started up FO4 to make sure nothing was broken yet. I got in this habit because when I first reinstalled everything (which was accidentally over the limit of esps/esms, since I had both NMM versions and Bethesda.net versions of some things) I had a CTD. By doing it bit by bit then checking, I have a much better chance of diagnosing which mod might cause that. It also helps to try to open up one of your old saves, because you'll get a warning dialogue showing you a partial list of what you haven't reinstalled yet. (Unless you're sure you no longer want those mods, hit the 'No' option when that warning dialogue pops up; you don't want even an exitsave without those mods installed.) My next step will be to make one or two changes to the .ini file so that things like Creation Kit work properly (instead of copying over my old .ini file), and then to copy over the retexture files I spent a few years making and accumulating, but those steps aren't necessary if you don't do things like CK or personalized textures. So, for anyone else with this problem: Good luck, hope this helps, and expect it to take a while if you have a lot of mods!