Is a multisplit air conditioner suitable for heating?

Yes, the pellet price is of course correct (leaving aside the free photovoltaic electricity since that should only be included in the calculation during the day and in summer) So pellets would still be the means of choice, I would just have to see if I can realize a pellet bunker somewhere so that the crap with the bagged goods stops.

The biggest disadvantage and perhaps also the main reason why air-to-air heat pumps are not particularly appreciated even in new buildings is the heat transfer via air. That is simply quite uncomfortable.

Radiators do that with the flow temperature as well. Most of it is convection. Advantage: somewhat quieter.

The mentioned air is also quite dry.

Since the air conditioning does not dehumidify when heating, the (absolute) humidity does not change.

I would not discard the plan immediately and initially install a single-split on each floor. 2.5 to maximum 3.5 kW is more than enough. Single-split because it is cheaper and more efficient. Install the indoor unit at as central a location as possible, or where it should be warmest. 60 sqm per floor can still be heated well by one unit if you keep the doors open and – as mentioned – operate 24/7. That also works cheaply at least down to -5° outdoor temperature.

It would be important to keep the pellet heating switched off as long as possible. This is like with a car: starting the boiler is responsible for a large part of the loss in efficiency. The worst thing for the pellet stove is the daily start in summer for 20 minutes of hot water production. This can also be avoided with solar thermal or warm water heat pumps.

Addition: Just to try it out, you can initially equip only the inhabited floor with an air conditioner. That costs around €3500. Leave out the radiators on that floor and try it out. In the worst case, you have a pleasant summer.

Sure, you could have gotten wonderfully warm with it, but you could also basically sit in front of the electricity meter with a bowl of popcorn. It was just really expensive. Accordingly, we only turned the things on as long as it had to be.

If you can't afford heating in winter, then you can't afford cooling in summer either. It uses just as much energy. So why did you install the 3 devices then?

Addition: To just try it out, you can initially equip only the inhabited floor with an air conditioner. That costs around €3500. Leave out the radiators on that floor and try it. In the worst case, you have a pleasant summer.

Very good air conditioners are available for €500-700, good ones from €300 up. You can install it yourself or know someone who knows someone.....

I know someone and order cheaply in Italy and do not come to these prices. Doing it yourself is forbidden in Germany and also not quite trivial. Those who have it done pay €1000 per kW. Currently probably even more.