Underfloor heating without a controller but how?

So if I’m fine with 20 degrees in the bedroom, do I just reduce the flow rate there a bit?

It’s not quite that simple (trial and error helps), because if 1,000 liters per hour are pumped through the house and 150 liters of that go through the bedroom, 850 liters remain (perfectly suited for the desired room temperature in the other rooms) for the rest of the house.
But if there are only 100 liters in the bedroom = less heat input through the underfloor heating, then 900 liters flow through the rest of the house, which may then get a bit warmer.

It's not that simple (trial and error helps), because if 1,000 liters per hour are pumped through the house and 150 liters go through the bedroom, then 850 liters remain (just right for the desired room temperature in the other rooms) for the rest of the house.
But if there are only 100 liters in the bedroom = less heat input through the underfloor heating, then 900 liters go through the rest of the house and it might get a bit warmer there.

I suspected that. But I think it should be doable. Thank you very much.

I occasionally visit someone who controls it only by the flow rate. During the heating period, I now refuse to sleep there because I freeze my ass off; otherwise, the room is unused. The owner no longer likes to adjust the flow control because he has to fine-tune it so precisely and afterwards can't get it back to how it was before. If you like that...

That simply indicates incorrect operation or just wrong settings. Or the person might just like it cold. I also know such people. The bedroom always has to be a maximum of 16° for them to feel comfortable. Other factors like a generally too low heating curve and/or poor execution of the hydraulics can also be causal.

But that does not affect the other controlled values?

As already mentioned, you should absolutely avoid making any more changes after the system has been balanced and the desired temperatures reached. Because each subsequent adjustment then throws everything slightly out of sync, not just the one room where you make the change.