Thank you for the offer, but I got it figured out and now I’ve got a separate network for Gree heatpumps (among other things) which would love to send all of their data to the manufacturer. Even the adoption process was pretty complicated where it required an account (of course) and wouldn’t connect to my separate wifi at all if I didn’t permit a temporary internet access trough it.
At the end the biggest issue was actually EdgeRouter which is runnign as glorified PoE -switch (I have older 24V passive POE unifi hardware still around). It should manage VLANs without any issues, but apparently even two separate IP addresses is enough to cause various issues on the thing. When I got that one out of the way the rest was pretty straightforward.