Has your car got Air con or not?
My next steps would be:-
Check coolant level in the expansion bottle when cold! - if ok.
Scan for faults and check measuring blocks for temperature readings.
If no access to VCDS
New thermostat
Temperature sensor -Check out this video on YouTube:
After that it's getting more difficult in terms of competency and tooling as your moving into heater matrix replacement and coolant system flushing!
The above assumes the interior blower fan is working?