I finally replaced the experimental #Homeassistant installation on the old #raspi 3b with a new, well-endowed raspi 5. What a difference. That thing can run!
I also got Modbus running and managed to get the #Lambda heatpump integration up, but the heat pump doesn't speak modbus yet, that will be enabled later on by a technician.
Now I only need a propane gas sensor with wifi for the cellar and with HomeAssistant support, then I am content.
... and can start on the doorbell stuff. ;)