Architect does not keep the schedule - what to do?

It's like raising children: you can only threaten consequences or punishments if you are also willing to follow through. For your architect, this naturally means firing him if the deadline passes. Otherwise, you make a complete fool of yourself and six months later nothing has happened. But you will still get a full invoice, because you are an easy target (to put it bluntly).
How about:
"Thank you very much for the draft so far, I have the following change requests, please incorporate them by October 10 so that the submission to the city can take place on ... . Any further delay is unacceptable and will be considered an immediate termination of the contract by you as the contractor. In this case, we also reserve the right to claim damages due to non-fulfillment of the contract."

Forget it. Apparently the OP has been

There were only so many appointments because there were constant postponements. So overall there were 3 meetings for which there were 21 appointments.

fooled eighteen times without a single reprimand threatening non-fulfillment due to refusal to work. This loss of face is never and under no circumstances repairable. I consider it infinitely hopeless to retroactively supplement a contract apparently formulated "informally, without notice, fruitlessly," to set a deadline for the fulfillment of the supplement, and so on. If the contract can theoretically be fulfilled at the end of time, the architect will demand a fee for his efforts according to the fee schedule and will get it. The contract can probably be terminated, but with the consequence of a complete write-off of the tuition fees.

However: until proven otherwise, I do not believe that so much misaligned (non-)action can be attributed to one-sided communication deficiencies.

If you set a letter with a deadline or it has escalated to the point where you have to set one... then you should have a lawyer draft it...

Honestly, the architect laughs about "I'm giving you a deadline until xy, after which I will terminate"... He definitely has a lawyer who will find a way why in such a case the services rendered + downtime + damages become due.

In my eyes, this is a common misconception... the client goes through the process once in his life, the lawyer maybe 10 times.
The lawyer of the general contractor or architect handles such cases 100 times a year. You won't win against them, especially not "quickly"...

If you set a letter with a deadline or it has escalated to the point where you have to set one... then you should have a lawyer formulate it...

The original poster intends to continue with the architect despite everything, so the first deadline can also be tried without a lawyer. The lawyer won’t do much more anyway but charges for an initial consultation (€226) plus one hour of letter writing (€120). If you want to get out of it as quickly as possible and don’t trust yourself to handle it, then go to a lawyer. But even here, you can first try to terminate the contract amicably yourself, following the motto: “Keep the down payment and we’re even.” We have no idea what sums are involved.

The architect laughs about that. He definitely has a lawyer who finds a reason why in such a case the services rendered + loss + damages become due.

Services rendered are paid anyway; the money is already gone with the down payment. Loss will be at most the agreed amount. Probably something in between, because the architect has done nothing. Question: How many euros are still outstanding and can additionally be invoiced? What damage does the architect have that he could claim? None! In the end, you have to weigh to whom you pay the learning fee: the architect or the lawyer. The maximum costs for the architect should be known; the lawyer could be even more expensive.

Thank you very much for your help. First of all, I really have to say, great forum. I did not expect so much input. Secondly: I took your advice seriously and wrote a letter, (checked by a lawyer friend), and sent it with deadlines (noted as very short). And lo and behold: 3 days later the submission plan was finished. (although not yet fully aligned with my needs and authorities) Whether this is a coincidence and the architect really had already worked or was awakened by the letter, I cannot say. In any case, there was a clarifying conversation with new deadlines that will lead to termination if not met. Whether these can be met or whether it will again bring postponement and delay remains to be seen. Thanks and best regards Nico