Execution planning - Can architects be given a deadline retrospectively?

Hello,
we have a problem with our architects during our renovation; the commissioning of the next construction phase "detailed planning" took place around mid-March. Until this point, we had no problems with the company regarding the timeline, so unfortunately !!! we did not set a deadline in the contract for when the service must be provided. And now the whole thing is dragging on - actually, none of the sub-items (electrical planning, lighting, bathrooms, drywall, tiles...) have been fully delivered yet [U]. For some points - despite multiple inquiries by email - we have not even received a first concept or anything.
For example, the company has already given us several dates by email regarding the lighting planning on which they supposedly wanted to deliver something, but none of these deadlines were met. We have believed them so far and have somehow let ourselves be postponed. That is why we have not set an official deadline yet.
However, these dates mentioned by the architects themselves are presumably legally not binding, are they? We would therefore like to finally set a legally binding deadline, after which we can reduce the invoice amount.
Questions:
What wording can we use to set a binding deadline by which the service must be provided?
What deadline is legally appropriate? (The architects themselves announced in an email on 21.5. that they would "finalize the planning in the next 1-2 days.")

I am very grateful for help! Regards

Lawyer. Otherwise, it will be tight here to unilaterally change a contract in your favor. It will probably result in a proper termination combined with the payment of a contractual penalty. Have you gone to the company / architect and sought a personal conversation?