We find the all-purpose room quite generous in terms of the square meters, but at the same time we (unfortunately) fear that 10m length is tight
No, it really isn’t, that is very tight, more precisely, clearly too tight with 37 sqm. I think it should definitely have at least 50 sqm. In my opinion, 12m length is the minimum for such a project. Especially with regard to the house size of just under 200 sqm. You absolutely have to take a look at that in a model house and then with your furniture ideas. It is also urgently necessary to take correct furniture measurements, e.g. sofa area, TV, etc., and to try out the distance between dining table and kitchen.
you can’t do without: large kitchen, large all-purpose room
Then 50 sqm are too little if it really is supposed to be particularly large/generous. You explicitly described the topic “large all-purpose room” as a must-have. But here you find exactly the opposite with 36 sqm!!! Even if you move the guest room upstairs, which I would find appropriate, you still wouldn’t have a significantly large all-purpose room, which was however your wish and must-have. The hallway is huge in area but, as already mentioned, not a cloakroom but only the word “cloakroom” in the plan. I also believe that it might rather be a rectangle and with the evidently difficult plot, the “inner values” first need to be found. Garage, parking space, children playing, vegetable garden.... something will be found for that as well.