Construction delay by the developer - Who bears the rental costs?

Reminding or issuing a reminder initiates a legal process that must be usable in court if you need to sue.
It won't be that simple: if there is no default clause (50€ per day or so) with some small print, then you shouldn't expect anything.
Provision interest is out of the question anyway. There's nothing for that because financing is your personal matter. The smart builder includes provision interest in their calculation of additional construction costs.
And my personal opinion: don't terminate too early. Parallel costs must be expected.

Our contract contains the following passage:

The contractor commits to completing the property ready for occupancy by 12/01/2024 and to full completion by 12/30/2024.
The start of construction is scheduled for xx.xx.xx.
The contractor shall only be responsible for a delay in occupancy completion if it is attributable to fault on their part.
The construction of the building project must be carried out according to the plans and the building description.

So that means we have no claim? For anything? At the time, we were pressured to sign the notary contract even though the floor plan and building description had of course not yet been adapted to us. After all, they said they are not a prefab house company, but that we could still adjust everything and make changes during construction. That is true regarding the equipment..
We now suspect a financing bottleneck due to another construction site, which is why there was no progress for weeks at times. Even after the demolition of the old building, nothing happened for months. And we have to pay for that now? Also, that we already had to take out a loan 10 months before the first partial invoice seems outrageous to me in retrospect. He had a financing confirmation after all...

It won't be that easy: if there is no delay clause (50€ per day or so) with some fine print, then you can't hope for anything.

No. That is wrong. You have a very clear legal right to reimbursement of all costs. This is confirmed by numerous rulings against developers.

Does that mean we have no entitlement?

As written, you have a legal entitlement to reimbursement of all costs. It is stated in the Building Code.

As written, you have a legal right to reimbursement of all costs. It is stated in the Building Code.

If delays are not caused by the service provider, the claim is at least questionable. When did you first notify the developer of a delay or doubt the completion date? As you write, today’s situation was foreseeable. Documentation would help enforce a claim. Were there delays due to permits, weather, decision-making on the house plan, material shortages? In my view, the client file plays a major role in the success of building through clear and immediate communication from contract signing to handover and remedying all defects. Sometimes I get the impression that people write in large letters “You can take advantage of me” on their foreheads through their way of (non-)communication. This pays off in construction, and that is why it occurs more frequently. Additionally: the client is also responsible for their moving planning and financing conditions.

You have a clear legal claim to reimbursement of all costs.

Being right is not the same as getting justice. But I don’t see that here at all. Against any theory stands, for example, the mentioned special equipment, which breaks the procedure in theory and prolongs the manufacturing process. This here is practically a free pass if something is changed:

A delay in the readiness for occupancy is only the contractor’s responsibility if he is at fault for it. The construction of the building project must be carried out in accordance with the plans and the building description.

I now have some quotes copied here to answer, but when scrolling back to the beginning I read something about a developer. But also about a contract for work. I don’t know if there are differences here, still the question: who is the client in your case? Are you buying a finished townhouse or are you the clients and building with a general contractor?

we are currently building a townhouse with a developer.

but we can still adjust everything and also make changes during construction.

Yes, that already breaks the facts of the construction process.

We were pressured back then to sign the notary contract, although the floor plan and building description had of course not yet been adapted to us.

That is not unusual. For example, with us the contract was expanded each time. That does not seem to have been the case with you. In my opinion, you should at least have kept that in mind. If I give pants to the cleaner who promises a standard cleaning within 2 days, but I point out a special stain on them at drop-off that absolutely must be removed, then I have to reckon with an additional time factor and cannot rely on the standard procedure.

We assume

That is not relevant.

why it sometimes didn’t proceed for weeks.

Craftsmen are not on site every day. We don’t know the background here.

And do we have to pay for that now?

No, you pay, if anything, your provision interest and your rent.

I also find it outrageous in retrospect that we had to finalize a loan 10 months before the first partial invoice.

He had a financing confirmation…

He would have had that even if you had all the money available without financing. How you raise the money for construction is not his business nor is it his responsibility. The latter I see with you to secure yourselves more, whether by inquiry, having the contract examined by experts, and your calculation including contingencies. In my eyes, as says, much can be written in the Building Code if one refuses to accept one’s share of responsibility. The mistake is: one almost always sees oneself as right, the others as wrong when suddenly one sees one’s money flowing away.

If delays are not caused by the service provider, the claim is at least questionable.

Although there is no legal obligation for the developer to avoid the delay directly (bad weather etc.), without any information from him he is legally in default. That is at least a fact. If delays are caused by the client, the developer should (must, for his own benefit) notify this.

The mistake is: you almost always see yourself as right, the others as wrong, when you suddenly see your money flowing away.

That is true. But according to the facts described (completely?), the legal situation is clear.