FINALLY was able to export this thing from within FCPX. Not sure why it works now, but here are more things I did:
I put Neatvideo's noise reduction plugin on a clip in a tiny project that normally takes half a minute to export. It took a half hour. So I figured that was the culprit and disabled all instances in the big project. Didn't work.
Disabled Filmconvert Nitrate. Didn't work.
Replaced a lone instance of the built-in noise reduction with Neatvideo. Didn't work.
Then I did a few things together that worked. (I should've tested after each of these, sorry!) Deleted a few titles, out of dozens, that were already disabled. Deleted a few adjustment layers, out of dozens, copying their lone hue/sat curves to the clips themselves. And "finalized audition" on the few clips that had them. Export then worked. And of course I might've done something else without realizing it.
Fwiw, exporting in Compressor worked throughout this ordeal, though it took nearly a half hour to do the small project with Neatvideo in it. The big project takes maybe 10 minutes? Can't explain it.