Drempel yes/no, which height

I have actually already thought about that too. Would you then make the planned 50cm "Drempel" smaller? Or use the area between 50-100 later. Sorry if I express myself so complicatedly.

Sliding doors between 75 and 1m high, behind them built-in shelves or carcasses with an appropriate usable depth.

Have the 50 cm threshold professionally done as planned. If you do the interior finishing yourself, meaning the floor coverings, then I would build a wooden frame at 1 meter (or variable where it fits best according to the furnishings, use, or need) and cover it with sliding doors. The area under 50 cm can only bear weight without insulation. It hardly benefits the storage space either. For a hobby area or guest room, the low area can provide some spaciousness if the room is otherwise quite small.

Would you then make the planned 50 cm "knee wall" smaller? Or use the area between 50-100 cm later.

As already said, very clearly: closed ("dead" / "sealed") only as drywall "baseboard" or pure vacuum knee wall max. 20 cm high. Under absolutely no circumstances would I lay the underfloor heating behind a later knee wall all the way into the very last dusty corner. Keep in mind that with a knee wall height of 0 and a roof pitch of 33°, you only reach one meter height after 1.54 m. So if you are considering a knee wall at this height, then leave this strip – meaning about one sixty centimeters – free of heating loops.