Insulated old building from 1921: Conversion from gas heating

So with 75° flow temperature, a heat pump is out of the question. You should aim for <40° at 0° outside temperature. And in general: surface heating systems are more effective: ceiling heating, wall heating, or underfloor heating. Since that's not possible in old buildings without ripping everything up, you can use type 33 radiators, which also have a larger heat emission surface than type 22, for example. This allows you to lower the flow temperature, which saves costs. I'm still just reading up on this myself, so I can't easily help you with your specific heating system. The first step would be to lower the flow temperature/the heating curve. This also improves efficiency with gas heating. Then you can also see if the insulation is sufficient for a heat pump and if you can get the flow temperature low enough. But that only works when it's cold.

The Federal Ministry for the Environment only proposed two weeks ago to slowly start banning pellets now. (Because of fine dust and CO2) So we are close to the end of this technology. There haven’t been enough pellets from Germany for many years and currently the Romanians aren’t really able to exploit resources either. So no. Pellets are not an alternative.

Heat pumps work in every house. Sometimes with more, sometimes with less effort. The heat load calculation recommended further above should definitely be the first step. In combination with a photovoltaic system covering ALL roof surfaces, it is the only future-proof heating system.

Go for it. It is currently even heavily subsidized!

Underfloor heating is now planned for the ground floor and new radiators are being installed on the upper floor.

You can also set the flow temperature yourself. If it unexpectedly gets cold again, you can simply turn it back down. It should work with the instructions, just film what you do so you can undo it in an emergency. Then you’ll also see what works with the current radiators. I think you can start at 50 degrees.
I’m not very experienced with insulation, but 20 cm sounds good and better than the 93 standard was.