As Paulster2 suggested, it was a dead cylinder, caused by a faulty fuel injector (see question update #2 and #3). As suggested by Justinm410, the injectors aren't too bad to reach on this engine and are definitely DIYable. I did have to take the intake manifold, throttle body and air intake off in order to have good access to the fuel rail, just to do the electrical troubleshooting.
The fuel injectors on this engine are pressed through the fuel rail and only held on by a little horseshoe clip, itself held on by 2 Phillips screws. Y'all can probably imagine what happened when I tried to remove these screws after 23 years and 298,400 km. Yup, the Phillips head got chewed up pretty good as the screw was seized in place. I used locking pliers to crack them loose and then unthreaded them. I replaced them with thread-matched bolts with a hex head, my favourite.
As for the electrical connectors for the fuel injectors, the plastic clips snapped off all 4, I now have zipties holding those connectors on. Nothing says DIY mechanic like zipties. I only ended up replacing the single faulty injector, I'll do the other ones if and when they go. I have my eye on #4.
And now, to go for an extended test drive, as I missed my little baby this week. :)