Still having the same issue as in >>2937682, except now it's happening basically every print. In five 5-hour prints, I've had one nearly-successful one and the rest have clogged within an hour or two.
It might have something to do with the fact that this is the first time in a while I've printed something at a smaller layer height than 0.2mm.
When it happens, even as it continues to try and print, I can get the filament to come out if I push it from behind the extruder with extra force. I might chalk it up to an old, weakened extruder, except that it takes a lot of extra force and the extruder is able to push the filament just fine when the issue isn't happening.

The symptoms sound like heat creep, but I can't find any physical evidence to support that. It still happens after dusting off the heat break (there wasn't much dust or debris on it to begin with) and both hotend fans are operational. When I remove the filament after a clog, it is not thickened anywhere along its length except for the plug from the nozzle at the very end, and the plug is no taller than the heat block.

The nozzle is old, but wouldn't a worn-out nozzle have a wider opening and thus be causing overextrusion, not underextrusion? The fan is old, but if it were spinning too slowly to cool the heat break resulting in heat creep, wouldn't the filament above the heat block actually be thickened?

Pic related on the right is the filament I pulled out of the hotend during one of these clogs. I pulled the one on the left while the printer was extruding normally, as a test. I don't see any difference aside from the plug on the one on the right being slightly taller, though still no taller than the heat block, so it doesn't seem meaningful.