Is there anything specific we should be aware of regarding the demand for the wastewater charge when tenants mix it with demands for window replacement?

Here, two completely different issues are mixed together, and it's important to keep them separate.

Payment for wastewater is regulated by §§ 71-78 on consumption accounts and advance payments. If tenants disagree with the statement, the objection must be made in writing no later than 6 weeks after the account has been received, cf. § 77. A general disagreement about the amount does not exempt them from paying — they must raise a formal objection, otherwise the statement remains valid.

Replacement of windows is a completely separate issue regarding the landlord's maintenance obligation. There is no connection between the two cases under the tenancy law — tenants do not have the right to withhold payment for wastewater as leverage to get windows replaced. The two claims must be handled separately.

Now that you have reached the demand for the remaining amount, the demand should solely focus on the payment — do not mix the window issue into it, as it may create doubt about what the demand actually requires.

The fact that tenants mention the Tenants' Association does not change the legal basis. It's a good idea to keep your own documentation (statements, correspondence) in order if the case ends up with the rent tribunal.