BSc thesis proposal: how detailed is the norm
I have gotten informal (but written) confirmation from a professor that the topic I suggested is ok, they even assigned some additional mentors...the thesis is not formally registered yet (it's mandated by university policy that registration needs to happen quickly/before substantial work) and I'm supposed to write a proposal.

Write as little as possible: The professor has informally agreed that the topic is okay and has assigned their resources. The professor is satisfied. Write something, check it is okay with the professor, and submit it. The professor is unlikely (imo) to be interested in the formal requirements, they (imo) just want you to get on with the work.