Is financing a single-family house feasible for us?

According to OP, however, everything sounds to me as if the basement is indispensable

Exactly, also a topic! With an "alleged" 300m², a basement can definitely make a lot of sense.

Although that is also an issue with: storage space inside the house or outside the living area. My opinion is probably rather in the lower third, but as already said, it always depends on the individual case. According to the OP, however, everything sounds to me as if the basement is indispensable, which in my opinion is not bad at all and I totally understand.

@OP: I can understand you with a party basement or hobby room etc.

Additionally, with a basement, everything generally looks tidier because there is more space available. I wouldn’t want to throw everything into the attic, park the other half in the garage, and as a result, park my car in the courtyard, as it currently looks with almost every new build.
Besides, the few small "storage rooms" you have aren’t really tidy either and offer little space.
Then the topic of laundry... For me also a very decisive criterion, which apparently for many is not a problem at all.

We don’t really have a proper "total price offer." That’s actually what worries me the most.

Of course But currently we celebrate at least once a month in the parents' double garage

ui that would make the hairs on the back of my neck stand up^^

what do people celebrate every month?