Switched this old laptop's window manager from a Wayland one to an X11 one and it's pretty obvious from using it these past several weeks that this was a major source of this thing's performance issues. Not gonna make that mistake again — I run a tight ship here, and after five-and-a-half years of dealing with an unstable rig I'm not gonna start adding possible sources of instability once the repairs are done.
(I knew it was a good idea to test out different environment setups before doing the repair. Less time troubleshooting once that gets going.)