BU withholds "confirmation of the builder" for BAFA funding

I don't have a lawyer and I don't want one either, it must be possible to get them to fill out this form without a lawyer.

Exactly, with the threat of a damages lawsuit if they keep ignoring it.

Yes, fill it out and just have it stamped.
You sometimes have to spoon-feed everything to some people so they can swallow it...

Did you do it like that? Did you also enter the amount of the costs? I haven't seen the invoice they got from the specialist contractor. They forwarded me the quotes from the subs once, but not the invoices. As far as I know, I don't need those either, since for us (instead of the invoices) the developer's confirmation counts.

I haven't done that yet (only applied), but you should have already received a cost breakdown from them with the application. I would now write exactly that breakdown in there, unless something really different was installed. Then send it to the builder with the request to stamp and sign it, and off you go...

The so-called "Fachunternehmererklärung" can only be issued by the specialist company. They are also responsible for the accuracy.

Possibly drive by (if not too far away), say: "I'll wait here until it's finished..." and sit down opposite the person responsible

The so-called "specialist contractor declaration" can only be submitted by the specialist contractor. They are also liable for its accuracy.

Possibly drive by (if it’s not too far away), say: "I’ll just wait until it’s finished..." and sit down with the person responsible

I received the specialist contractor declaration from this very person without being asked. I am missing the "confirmation from the developer," basically the counterpart to the invoice for people building with a general contractor.

Is there a deadline you have to meet for submitting this form?

If yes, I would do it as 11ant suggested, write a letter to your general contractor with a deadline, asking them to send you the completed, stamped, and signed form (to be safe, set the deadline you give the general contractor to at least one week before the official deadline expires) – with the "just for completeness" note that if you do not receive the form and therefore lose the funding of X euros (feel free to insert the exact amount), you will recover the money from the general contractor. And if this is even regulated in your general contractor contract, you can also refer to that again.

Send the letter by registered mail with proof of delivery and you’re done. And if you’re paranoid, you can also find a witness for putting the letter in the envelope and dropping it in the mailbox...