I got it working in the end.
I now completely migrated from OZW to ZwaveJS (w00t! 
)
ZwaveJS can’t set the valves to ‘Manufacturer specific’. The details are described in this item. (it’s about another valve, but the Eurotronic valves are also mentioned)
Missing Manufacturer Specific mode and valve control for Aeotec TRV · Issue #45703 · home-assistant/core · GitHub
It boils down to the fact that the valves don’t advertise Manufacturer Specific option to zwave-js. To ‘resolve’ it, either Eurotronic needs to provide a firmware update. Or zwave-js needs an update for ‘device specific quirks’ which would allow to push settings to the valve that the valve itself doesn’t advertise it can handle.
I ‘resolved’ (more a delicate workaround) my issue by setting the valves to ‘Manufacturer specific’ using the OpenZwave addon. (didn’t need the integration of course).
So what I effectively did:
- Disable ZwaveJS Integration (which also shuts down the ZwaveJS addon)
- Start MQTT broker (I don’t have it running by default)
- Start OpenZwave addon
- Change the settings to ‘manufacturer specific’ using ozw-admin (the gui for OpenZwave)
- Shut down OpenZwave addon
- Shut down MQTT broker
- Reenable ZwaveJS Integration (which also starts up the ZwaveJS addon again)
The ZwaveJS integration also provides a ‘numeric’ entity which controls the radiator valve’s position.
The only I need to keep in mind, is to not touch the ‘Mode’ in HA, else I can’t change it back to Manufacturer specific without the above workaround.
Edit: I have put my setup under the Community Guides:
Eurotronic Spirit radiator valves with external temperature sensors in ZwaveJS - Community Guides - Home Assistant Community (home-assistant.io)