Warm bedroom in the house with ventilation including heat recovery

As already said, it is not the ventilation but the house itself... and in summer it actually gets even worse... much worse actually... if you already find 21° bad.

For the ventilation, I also recommend just letting it run at the minimal setting at night.

I know the nagging even without the ventilation system. Somehow the doors are always open here, but then complaining when it’s too warm in the bedroom and the kids’ room. If I turn the heating down upstairs, of course the warm air from the basement pulls up to the upper floor. The result is the upper floor is just as warm, basement cool -> nagging again

Well - the annoying topic again.
This is something you can hardly influence yourself.

Measures:
year-round short pajamas or similar
use the lightest blankets year-round
air out vigorously for 5 minutes before going to sleep

Well - and if this doesn't help: My wife likes to open the windows during the night - to my dismay (why waste the energy).
I have programmed our heat pump so that it does not run between 10:00 PM and 7:00 AM. At least this way we don’t heat for nothing when the window is open during sleeping hours. If the windows are not opened, there is no significant heat loss during this time due to the insulation (this would then be interesting in winter).

Nothing helped me except a split system... but we also have the bedroom facing south

We air out once more late in the evening before going to bed. This usually lowers the temperature to 19.6 degrees. In my opinion, still too warm to fall asleep, but we’re gradually getting used to it. My men have only been sleeping in T-shirts since the start of the heating season, our female teenager still in a long dress. We solved the problem with the "draft" by moving the bed a bit. Just try out what helps! There won’t be a one-size-fits-all solution here.

nothing helped me except a split system... but we also have the bedroom facing south

I also believe that the 5 to 10k for air conditioning in the living room and bedroom in a new build is very, very well invested. Two summer days at 40 degrees and the room temperatures become unbearable.