Thought I might try a few random experiments today - removed the airbox and serpentine belt (let it dangle underneath the car) and tried starting it up.
Guess what - engine runs perfectly with the belt disconnected!
And when I replaced the belt, back to stalling immediately and idling terribly.
All the pulleys and tensioners seem fine.
So it comes down to a component driven by the belt causing the problem
- water pump (unlikely? only replaced 2000km ago, mid 2020)
- alternator (seems most likely to me, i did change the regulator in it in 2018)
- power steering pump (hopefully not)
- airconditioner compressor (unlikely, as it freewheels and disengaged/not turned on)