Banks conduct so-called Konditionsanfragen when reviewing mortgage loans, and these are schufawertneutral; moreover, banks can only see these inquiries for 11 days.
They do not have a negative impact, and I don't see how you would come to think that all banks assume you are only requesting a condition/offer from the respective bank.
If they do see this, then the bank should actually offer you an even better deal and not, as is often the case, first present a poor offer in order to improve it later because you question the bank's offer with a significantly better offer.
For me, there is no reason not to sign with DEVK if the conditions and terms fit, especially since insurance companies like Allianz, Ergo, AXA, Unversa, and Hannobersche always have the best offers for terms of 20 years or more.