Construction costs are currently skyrocketing

Ah, slab ;-)

For us, only a white tank would have been allowed according to the development plan :(

Is it possible to lay an empty conduit through this free entrance in advance, to be prepared for the possible case of a different house entry later?

Yes - all the inserts are available to buy separately... I'm laying the free line to the cistern (power supply for pump)

Regarding the topic of multiple cores. I have the gas inlet free, as I have an air-to-water heat pump. Is it possible to lay an empty conduit in advance through this free inlet to provide for the possible case of a different house entry later?

Doyma says yes, that unused entries remain in the delivery state. For the multiple core for the basement it is like this: An empty conduit is attached as an extension to the outdoor unit and the stuffing box flange of the indoor unit remains as it is (namely sealed). You can buy a different stuffing box flange from Doyma if needed - depending on which one you need. Then route the connection through the previously extended outdoor conduit and screw on the new stuffing box flange, done. Whether this works the same way for the multiple core for the floor slab I cannot answer. The question is always what you want to connect later and where you then lead the end of the empty conduit which does not reach the multiple core. So near the public ground or somewhere in the garden because a cistern and pump are to be installed there later? You have to think carefully about that. Because depending on the connection there are requirements for the depth of the connection. Telephone may be laid 50cm deep, water had to lie 1.2m deep for us. This must be taken into account when pre-installing empty conduits.

Our general contractor also installs a Doyma multi-circuit square with 4 entries in our base slab. My plan was now to manage all the electrical lines for the garden (garden house, lighting system, cistern pump, driveway gate) through the fourth, currently empty, entry. But that is already possible, right? Or what do I need to pay attention to?

These are ring seals. To my knowledge, you cannot simply run "multiple" cables through one duct, or there are sealing inserts for laying different cables in one duct. However, you could also simply lay a seven-core power cable if that is sufficient in terms of phases and branch it off outside the building.

With the right stuffing box gland it is possible, as mentioned, the problem could be the sealing of the other end of the empty conduit, depending on where and how it is then positioned. Here you can see how several cables - also of different diameters, come into the house through a stuffing box gland.

* Edit
When they installed the water pipe for me, they forgot the rear seal which is inside the gland during installation, luckily I noticed it because it was lying around :-/

Something about construction costs again ^^ The ton of sand 0/2 washed currently costs €18.90