Now everything is very reliable; I'm using:
- WiFi (aka Shelly) - thanx to a very good router (Fritzbox).
- Zigbee - thanx to a bunch of IKEA repeaters.
- Hubitat - thanx to avoiding Rule Machine.
The only "problem" are my Google Minis: Every some weeks a random one is loosing the connection to Google Home and I have to "reboot" (i.e. briefly interrupt the power supply of) that one Google Mini.