Domoticz

Simple question (I think)

What is the best zigbee connection to buy what works with domoticz on my Pi. I have the Pi fixed in the basement and the distance to the centre of the house is about 6/7 mtr. What would be a good setup to create/buy.

Currently I have an sonoff zigbee bridge with tasmota, this works for my PIR sensors but I can't make the thermostat work so I want to try it over zigbee2mqtt but want to buy some decent hardware. PLease advice

You can also add an extra zigbee router in between bridge and thermostats. Every 220V powered zigbee device (bulb, power switch, dimmer etc) can act as a router, but there are some exceptions. So check first.
Using just another Zigbee coordinator (Bridge) will not always solve your signal level issue.

I don't think we are on the same page, I want to leave the current setup as is. This is setup with rules in tasmota to send mqtt messages to domoticz.
I want to add an extra zigbee system but then use the zigbee2mqtt in domoticz to see if there is more functionality to control an thermostat.

The issue is that I have one thermostat eurotronic that I can read the data from, but have no way of controlling it. If I can control it then I can buy more of those for the whole house. If not I have to find other ways for controlling the heat in several rooms.

Then you look at wiki page https://www.domoticz.com/wiki/Zigbee#Zi ... n_Domoticz
All Open Source (USB) Gateway/Hub integrations are capable of your request in controlling your Thermostats.
I personally have a Zigate, I do not know what the best hardware is for a Zigbe22MQTT coordinator. But Google will be your friend