Underfloor heating and air-to-water heat pump in new construction: am I going to have problems?

Unfortunately, absolutely useless, except that it somehow gets a bit warm

I will agree with the GU to lock him in on - house will be warm - max. 4-5 cycles per day in winter - annual performance factor > 4.5 (calculated is 4.9)

And that is exactly why you get a buffer, because you expect all that everyone wants. Only that you already know beforehand that the setup is stupid. With that knowledge, you don't even have to show up in the other forum in 1-2 years to ask if the consumption can be reduced by adjusting settings.

I claim, however, that especially the switching frequency will not be achieved with the buffer in the lead. The question is whether that is really so bad. In the shopping center around the corner, an expert is circulating the idea that today's heat pumps don't mind switching a bit more either.

I also have a Vitocal. Although brine and not air, that should not matter for the question of buffer or not. To my knowledge, Viessmann only specifies having a small buffer in the return flow. There are even suitable Viessmann boxes for that. The 200l in the supply flow almost certainly does not come from Viessmann. You can try googling planning diagrams or ask Viessmann directly and confront the heating engineer with it.

Everything you criticize in the original post, you criticize completely rightly. Circles are spaced too far apart in length, distances too large. Insist on a room-specific heating load calculation with the specifications known to you. The buffer must go. ERR then just take ones that are open without power. Then simply disconnect at the end yourself.

Is there a room-specific demand calculation?

If necessary, you can also have a "bypass" installed with manual 3-way valves for the buffer. Then you can simply remove the ERR after handover and take the buffer out of operation. That way you save a lot, and the general contractor heating engineer gets their buffer. The heating engineer should be able to install that for little money.