Kabel Deutschland or Telekom or both

Glass replaces the traditional connection.
(All-IP has already been the standard for new customers for a while, but it's not a big deal)

I would also clearly choose the Telekom connection. VDSL 50 is more than enough for a normal single-family house for the next few years. By the time that is no longer sufficient, Telekom will have switched to Vectoring/Supervectoring and then 100/200 will certainly be available.

I would only be worried in areas where Telekom already offers only <=6 MBit, but that is not the case here!

We have decided on the cable connection from Primacom (from €29.99) with TV, Internet 20, and a flat-rate landline. Nevertheless, we are having the Telekom connection installed as a precaution and will probably also install a provision (cable or empty conduit) in the attic for a satellite system.

We will have both installed, Kabel Deutschland and Telekom. The reason for this is that Telekom can currently be forgotten here with DSL 2000. Kabel Deutschland offers (over fiber?) a multiple of that. So we will - even if it costs more - have both installed to remain flexible in the future. We will initially go with Kabel and sign the contract there, but we also want to have the option of alternatives later.

The competitors of Telekom use almost all Telekom lines, thus assuming their existence. Vodafone did so until recently as well, which is why they made the strategic acquisition of Kabel D (which, however, is not active in all federal states - where it is not, Telekom usually remains their line landlord). Fiber optic providers mostly lay their own lines but usually only develop new construction areas initially.

Yes, just like with the railway. The privatizations of the two authorities Bahn and Telekom were not thought through to the end. The networks should have remained public, just like the roads.