Domoticz

I can control my thermostat via domoticz.
However, I want to go one step further. I want to be able to operate this via mqtt. I want 1 command that increases one degree each time and 1 that decreases one degree each time.
I have now reached the point where this command works to set a setpoint via mqtt with:

mosquitto_pub -h localhost -m '{ "idx" : xxx, "nvalue" : 0, "svalue" : "15;0000.00" }' -t 'domoticz/in'

but I want to be able to increase or decrease with a command without entering a value. Is this possible and how does it work?

No, that is not possible without scripting (get the current value and do the delta plus or minus).

The Domoticz user interface does the same: Get the current setpoint and then do the delta with plus or minus arrow.

that's to bad. Can you help me get started? Scripting? Do I have to use LUA for this? I've never written a script myself before, so that seems like quite a challenge

What is the use case?
From what script/application do you want to do the mosquitto_pub mqtt command to up/down the thermostat?
That script/application should first read the current value (it should be readable in mqtt domoticz/out) and the send the domoticz/in mqtt command setpoint to be.

I ordered a button plus
https://button.plus/

This works with MQTT.
I want to remove my thermostat and and this is where I want to place this display. With the button plus I want to be able to turn on my thermostat with two programmable buttons. One button can be set warmer and one to set it colder.
how could I achieve this (with my limited programming knowledge)?

Ask button+ to support mqtt auto discover protocol and configure it to be a thermostat. Then Domoticz will recognize it and no programming from your side is required.

How does the button+ knows what the current setpoint is? It should need some internal configuration rules to send Pröpper mqtt messages.

Else you have to wait for a button+ owner with programming skills to do it for you, or dig in the programming world yourself

A in between solution is the mqtt mapper plugin where you can configure non compliant device mqtt protocols to create and update devices with mqtt protocol of the button+