Sorry you'd not had any replies.
Some of us on here watch all new posts, but others only watch the forums for their own vehicle. I've not been on here as frequently as usual, but I'm surprised the other 'regulars' have missed your post.
A slipping belt is often a slack belt, but the sprung tensioners on most these days mostly remove that possibility. So the next thing, as you've guessed, is one of the driven pulleys seizing.
Ideally, you need someone else to start the car, while you peer into the space where the belt sits. Then hopefully you can see which pulley fights, either from whichever one is not rotating, or from where the 'smoke' is generated.
Your description says it drives the aircon, power steering and alternator.
Aircon pumps can seize sometimes. Most systems do not energise the pulley clutch until after engine start, so that should be ok, as any issues should show just after start, not immediately.
Power steering pumps can also seize, so ensure the steering is straight ahead on startup, so it is asking for minimal pressure.
Alternators occasionally crack their casings, which can lead to tight shafts. Might need a mirror on a stick and a good torch to check that over.
Like said above, if you can, remove the belt and turn each driven pulley by hand. Alternator and aircon should rotate easily, and quietly. You are feeling for roughness as well as ability to turn. Steering pump will have resistance due to the oil inside, but its rotation should be smooth. You may be able to feel the pumping pulses, but there should be no rough feel to the bearings. The tensioner is the most likely culprit, and if so, will feel rough when turned.
Of course, it will need a new belt too.