Corsan Shower Panel: Thermostatic Mixer Issues, Removing & Descaling Thermostat, Repair Options

Hello,
I have a Corsan shower panel with a thermostatic mixer mounted in the bathroom. Warm water has stopped running for several days. I suspect the problem lies with the thermostat: / I tried to unscrew it to soak in a descaling agent, but I have no idea how to do it. The panel is probably the older type, because the back does not have access to the head, and in the front after removing the knob and the cover looks like this:

The question is how to unscrew the thermostat? The plumber I called claims that there will be a problem with buying a new one and that repair is expensive at all and waiting for parts for about 2 weeks: o I suggest switching to a faucet with a mixer at all, because thermostatic valves often go bad. I would be grateful if any of the smarter users than me advised something.

Well, it doesn't have to be the thermostat's fault. Just in case I check the hot water pressure hose for blockage. I have repeatedly changed the clogging of strainers and non-return valves with such a battery.
As for untightening, first unscrew this nut from the back and the tube going up (the metal one) and remove it all forwards.
After unscrewing, the thermostat is fixed with a screw and is removed from the housing forwards.
Below a photo of the thermostat itself. Plus, it looks like the whole thing.

Thanks for the reply, I will try to get started tomorrow

Are there any strainers inside this element from the first photo?