Renting, ownership of a condominium, or just building after all?

Well, let's give a shout-out to the building savings contract; as long as you know the overall effective interest rate of the TA/building savings contract, you can perfectly lend it with an annuity loan. (And that is not interest 1 + interest 2 / divided by 2).

We wouldn't have gotten our 29 years any cheaper because our final financing bank couldn't offer anything over 20 years; at our local Sparkasse, it even ended at 15.

But as it is in the initial post, it is not clear that it somehow fits together. (Conceptually)

Yes, in very few cases the building savings contract is a sensible addition, especially with very long terms...