Done! Thanks!
For anyone who needs the story: I logged out of KDE and then back in but as root, deleted both existing ordinary users (jbb and jeff), chowned everything that was owned by 1001 to 1000,
rebuilt and got a warning about NixOS not wanting to touch /etc/passwd, futzed around with a few more manual configurations (I didn’t take notes, sorry! but it was to bring /etc/passwd in line with the other changes), rebuilt again and got no warnings, and now I can log in as jeff=1000 to KDE with Plasma 6.