New construction - Telephone/Internet connection

Well, whether one of the two lines will arrive on time is, I think, both uncertain. You have now also made me reconsider it. We get up to 100 Mbps with Telekom at our place and fiber optic then of course 300-1000 Mbps (download).

However, you have to take a fairly high contract to include, for example, a landline flat rate. I find that nowadays unreasonable, even if you hardly need it.

A real GF connection is also not exactly cheap. What use are high download rates if I have an upload of 40mbit +/- 10. If anything, only a connection of 500/250 or more is really interesting.

You can't say that so generally. To use the 250 Mbps download, it takes far less than 40 Mbps upstream. It's clear that more upload = better, but I personally rarely upload large to very large amounts of data, whereas I download practically daily. Of course, the up/down ratio is technically determined, but it looks even worse with cable. That doesn't make a DSL connection equally uninteresting for most people.

I have 50/100 fiber optic, which is usually enough. Many servers on the network manage less, and for websites that is already oversized anyway.

The ratio is not technically necessary with fiber optic. Fiber optic can provide synchronous upload/download speeds. Even with DSL, this is technically no longer necessary compared to before,

You mean symmetric, not synchronous. And yes, that is clear. I was referring to ADSL and cable, and the standard determines that. SDSL is a different matter, but that is probably of little interest to any private individual.