Liability for various defects after acceptance of the property

Chamfering the lintel is a good idea and can practically be done like that. The company will not do it because the structural engineer will be horrified if someone wants to saw into a lintel afterwards.

I actually see the defects like this: negotiating an appropriate price reduction will be the best option in everyone’s interest. Then air out the grinding spots yourself, set stops, and live with it. (or get a friend to help if you yourself are not very interested in handiwork).

Standard advice: invest 300 to 500 euros and have a specialist lawyer for construction law explain what you have signed (special tip for readers: this is even better before signing, but also afterwards IMHO very well invested money, you will become smarter in the process).

Then you will have a clear understanding of the contractual side.

We had an expert with us who created a list of defects on-site [...] In any case, we refused to take over.

That’s good for now! The lawyer will advise what you could do with the list of defects. Then you will see further.

One should assume that the developer is advised by a specialist lawyer in any case. IMHO you save time and nerves if you immediately bring yourself onto an equal footing. Whether you then respond with or without a lawyer’s letterhead can still be decided.