I wasn't aware of this functionality when I wrote my post, but it defently works fine. Already tested.
When you disable automatic temperature control, the valve can be set in any position.
Thanks for checking!
So in a nutshell one has to do the following thorugh API:
1. set "/settings/thermostats/0/target_t.enabled" to FALSE (disable automatic valve control / "manual override")
2. set "/thermostats/0/pos" to desired position
3. set "/settings/thermostats/0/target_t.enabled" to TRUE (enable automatic valve control / return to the state before the "manual override")
right?
If you have a TRV around, could you please help me by checking if the MQTT topics allow the same?
(I haven't ordered any TRV yet, as I first want to check if the valve can be manually controlled through MQTT / HomeAssistant).