I did some digging, and found some other potential root causes from other people having had the same issue:
- coil
- fuel pump
- fuel lines (cheap enough to replace, so I'll order some suitable ones)
- Distributor cap (I have a spare one, so easy enough to check)
- Vacuum valve (?) between distributor and carbs
Some other, maybe related issues, I just remembered: Previously, before having changed the relay, it smelled fuel, now it doesn't anymore. And the fuel tank was replaced like 1k km ago. The new tank sometimes leaked from the filler cap when full, now not anymore. The old tank was rusted through.
Did a test with the fuel tank open, didn't change a thing. Slowly, I am ruling out the carbs. Cold performance and the lack of issues can be explained by the cold start / temp thermostat enriching the mixture. Under load it is better, but nowhere near what it was initially. So I slowly suspect fuel (not getting enough) or something ignition related. Only thing I didn't check on the carbs is the float chambers. I'll tackle distributor cap and fuel lines next, maybe it changes something.
Man, fixing faulty stuff is fun. Diagnosing the fault not so much...