Apply for cable and/or DSL connection

If I can choose between the three options:
- then fiber optic clearly wins. Higher speed + better stability.

Only DSL and cable available:
- If the same speed is available over DSL and cable: DSL because it is not a shared medium
- if there is a significant difference between DSL and cable, ask neighbors how stable the cable internet is and choose that. I myself have not had any problems with cable internet, but I neither live in a big city nor in a cow village.

Short-term, sporadic outages occur with all variants and cannot be prevented.

From painful experience, I can also say that it is wise to be a customer of the owner of the line. I used to have DSL with Vodafone and once had problems with the connection. Paraphrasing Michael Manousakis: CATASTROPHE. Meanwhile, we are with Telekom and in the current house, of course, there was also an outage because the line from here to the DSLAM was faulty. I called in the evening; the technician was there in the morning and fixed the problem. During the Vodafone incident, there was also someone (also Telekom), but he had absolutely no desire and no time. Customers are clearly prioritized (and rightly so, I think). As long as everything works, you can even have DSL with O2 :) But woe if something ever happens... and it will almost certainly happen at some point.

Yes, or a customer with a notice period of one month. Then there won't be any problems at all...

From painful experience, I can also say that it is advisable to be a customer of the owner of the line.

No. You can get DSL in original Telekom quality on Telekom TAL from the nfon subsidiary "Deutsche Telefon," but among other things with significantly shorter fault clearance times than from the TAL landlord himself. With DTS, I call Mainz and speak with a competent employee. With a fault on the same line and a contract directly with the network operator, I would call an 0800 hotline, tell my story, get kicked out of the queue, end up with the same hotline number in a distant federal state, tell my story again there, and with luck the fault would disappear faster than someone cared.

Whereas cables as a "shared medium" more frequently have to deal with overloads than copper.

The topic of "oversubscription" does not only concern the so-called "last mile" and, unfortunately, is independent of the type of connection. Sometimes it is also the leased long-distance networks of providers without their own infrastructure that create a bottleneck. Therefore, it is advantageous to order directly from the network operator – ultimately, this also justifies a somewhat higher price. Only with a fiber optic connection is the likelihood of oversubscription very low.

By now we are with Telekom

If you are satisfied with Telekom's peering, then there is nothing against it. I was not, or rather find it a disaster, and therefore switched to O2. O2 is worlds better, and with O2 I get my own IPv4, which is usually no longer available with the other resellers.