In the meantime, I built a workaround in HomeAssistant. I created two custom events, one when the window is open for more than 1 minute and one when the window is closed again. When these events trigger, I call URLs to either deactivate the automatic temperature control and close the valve or respectively activate the automatic temperature control and thus switch back to the last time based profile.
It's not product grade, not elegant, but as a workaround for a single D/W and TRV combo, it works. And I will only buy more TRVs once the real solution works...