How to find an object (house/apartment/land) nationwide in Germany?

Polish border?

Exactly, Görlitz. Plenty of retirees.

As you can see on the Seedomaine website, there is a farm shop and village shop within walking distance. Likewise, doctors, beverage markets, etc. are not far away. The local Edeka also delivers... Before suspicion arises: I personally do not own any property there. But I have a few acquaintances and also friends who have bought apartments there (as holiday homes). However, these are used exclusively by them and are not rented out. This is probably the case for most owners. Therefore, I have been on site several times and know the circumstances a bit. Most neighbors are therefore rather sporadically present. Other apartments in the Seedomaine are also rented out. However, this is certainly the nicest one available for rent. The rent is reasonable for what is offered. Alone the garage space could probably be very lucrative if sublet. The cellar and wine cellar are huge. Sauna etc. can be used jointly from the hotel. There is supposed to be a private beach or lake access and a private boat dock. There are several restaurants nearby and you get to Lake Constance quickly. There are many great areas. In none with good infrastructure will you be completely alone. You can also take a look at Bad Dürrheim. Beautiful spa town. There is certainly quite a bit to rent here as well. Some new buildings are being constructed.

Exactly, Görlitz. Retirees galore.

I think it is one of the most beautiful cities in East Germany. But I believe it is not the quiet place that the OP is looking for.

As for hobbies, definitely making music. That also includes occasional recording with a microphone. One of the reasons why I sometimes need absolute silence.
My wife runs, among other things, a cooking blog, so she needs a well-lit photo room. And a large kitchen.

Rügen, the island, is hardly on my radar at all; it is so large that basically all the island advantages are gone again, but the disadvantages are still there.

Specialist doctors, that is certainly a good hint, but as long as you don’t know what you will need later, it’s hard to plan accordingly. It doesn’t help if pretty much all specialist doctors are quickly reachable except for the one you need. And I don’t know that one yet...

I also tend more towards renting than buying and actually rather not building at all (maybe I’m in the wrong forum?). I think regarding timing, I am probably already too late for building, I should start thinking about buying now, and for renting it’s too early to seriously search (goal: summer 2021).

Regarding hobbies, definitely making music. That also includes the occasional recording with a microphone. One of the reasons why I sometimes need absolute silence. My wife runs, among other things, a cooking blog, so she needs a well-lit photo room. And a large kitchen.

Then a small house with a music room in the basement. It can become an excellent absolutely quiet studio. During that time, your wife can also gladly make noise.

P.S. You have been talking for four pages about what YOU must have. What does your wife want? Does she perhaps also have an opinion about YOUR plans?

With your requirements like making music, I would rather look for a small house to rent. I think with apartments there will sooner or later be problems.