Are professional painters worthwhile? Or do it yourself?

Well, if we repaint next time, for example the kitchen in about 5-10 years, we'll probably do it ourselves too. But painting the whole house for the first time is beyond our capacity in terms of effort. Cheap painter's tape is often crap, by the way, because it constantly tears when pulled off. But if you don't care about that, you can save money there as well.

Why do you actually assume that if you do it yourself, only the cheapest stuff will be bought? Buy good equipment and high-quality paint and off you go. Painting a wall white should really be doable. Whether you have to putty and sand yourself if you've never done it before is another question.

And... who will fill and sand the surfaces for you if you want to do the painting yourself? Try to find that person.

I only obtained quotes for spackling in Q2, Q1 was already available. No one was bothered by that.

Masking door frames etc. is not rocket science either. Just recently, a big discount store had such crepe tape cheaply

If I'm going to do it myself, then at least with decent material. Anyone who has ever worked with, for example, purple Storch tape will never touch such a discount store product in their life.

with the roller and then not getting so close to the other wall

Depends on the roller. There are some from Storch that also get into the corners.

We have now properly stocked up on rollers, brushes, and other tools at the specialist store during the modernization (for a total of under €100). Despite using many rooms and several colors per room, they are still like new.

And... who is going to fill and sand the surfaces for you if you want to do the painting yourself? Find someone for that.

The plasterer, of course. Just order Q3/4 directly or afterwards.