I have absolutely no experience with Tado Smart Valve, can you explain a little bit how you are controlling with through Home Assistant ?
I think what you’re looking for can be easily achieved with template value or even HA conditions.
An automation triggered on this the temp attributes change
Then a condition if Tado → attributes → temperature → int > 21
Then set it to 21
I cannot go more accurately since I don’t know how you’re used to control it.
Hope this help though
EDIT : would be nice if you could show the device config if you need more help on this → development tools and search for Tado’s device