The developer does not allow any self-performance before acceptance

What does your house construction contract say? There is surely a paragraph about self-performed work - which announcements, agreements, approvals, preliminary acceptances from each side must be fulfilled.

What does the site manager say about the quick way? Don’t make a big fuss: can we quickly repaint Room A and Room B? Done. When the whole house is painted after Christmas, what should happen? If someone comes at you rudely: it was agreed with the site manager.

He says he would be liable for it or everything on which we provide own work would be considered accepted.

I can understand and comprehend that, but if we still want to do something beforehand, is there no possibility to draw up something in writing that it is done at our own risk?

You answer that yourself. Just say that you are aware of the risk of possibly having to repaint entire walls.
Of course, he can still be stubborn.

Our developer

Developer or general contractor/construction company? Are you buying a piece of house including land?

I don't understand! WHY is it so hard to answer questions that could be important for a response?
I would say: if the dialogues with the site managers and so on go the same way, then you simply don't achieve what you actually could achieve.

I can understand and relate to that but if we still want to do something beforehand, is there no way to put something in writing that it happens at our own risk?

Ask the company. That is a question for the construction company.

What I don't understand is that our neighbors who build with companies like Eco or Team are allowed to do everything whenever and however they want.

Eco and Team are GCs, not developers. Quite simple.

Our contractor has allowed us to do this without any issues after consultation. However, he said that he usually conducts a written partial acceptance for the areas.