Combination of air-to-air heat pump, air-to-water heat pump, solar thermal and photovoltaic system with storage

The roof is oriented towards the south with a pitch of 25° - I know, not optimal, but the gable height is limited by building regulations. It gets tight with a steeper roof having 2 full floors. The system will have 24 modules of 340 heat pump each. The next "expansion stage" from the house manufacturer is 10kWp, but apparently they no longer fit on the roof (~12m ridge x 5m rafters)

You have an ideal south-facing orientation and the roof pitch is also close to the ideal point. If I were you, I would try to fully use the south side. If only 24 modules fit, then take more powerful modules (e.g., ones with 380 heat pump) or ask about modules with other formats. Due to the low roof pitch, the north side can also be interesting; I would at least have that simulated. You won’t get cheaper than a photovoltaic system within the framework of a new building.

You won’t get a photovoltaic system cheaper than as part of a new build.

Why? Because of the scaffolding?
All solar installers I contacted wanted to start only once the house is already standing.

Yes, I had already considered the north side as well. Or a south facade system for the winter - just because of the aesthetics, it's worth considering :D
2x empty conduit will definitely be laid, so the north side can still be easily retrofitted later

The scaffolding remains after the shell construction until the completion of the exterior walls (plastering / facing). During this time, the solar installer can become active at the latest.
If you are building a very large photovoltaic system and, for example, have planned the roof covering with clay roof tiles, you can consider a downgrade to concrete roof tiles. This saves money and afterwards, you won't see much of it anyway.
Our photovoltaic system will have 26.6 kWp, so not much of our expensive clay roof tiles will be visible.
If I had known that beforehand, I would definitely have chosen cheaper roof tiles.

But do you really need such a full scaffold? Or is the scaffold for the solar system really that expensive?

But do you really need such a full scaffold? Or is the scaffold for the solar system really that expensive?

Definitely much more expensive than if you can use the scaffold that is already there anyway. Then it’s just a few hundred euros, but you could buy something nice for the garden with that money, for example.