[Klaus]

I'm still not sure whether this can be a really sustainable investment (reads: I would really use it beyond the initial rush).

I'm also wondering whether I shall start with the basics or - because if this is going to be a thing - with something that provides a little more.

A basic thingy would be a "Star Adventurer Pro" 
https://www.youtube.com/watch?v=JIwuidp9rCs
or maybe this
https://www.skywatcherusa.com/products/eqm-35-mount

I still have the old EOS 5D II which I could convert to Astro - although I may do that later on really.

I suppose that the start would be about some ultra-wide landscape shots and 300mm-ish nebulas.

Now ... can somebody talk me out of this to order to keep my sanity or tell me that this is not a stupid idea? ;-)

My condolesences to your financial freedom.

We are talking about an (additional) investment of USD800-1200 here  ... or the price of a mid-grade system camera ...

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1. Do you have easy access to place with  low humidity and/or  high altitude?
2. I assume that you can find place with less light pollution, Am I right?

Well, I'm living in Sydney and within the city limits, you can forget about astro photography, of course.

However, I don't really have to go far ... 
https://www.lightpollutionmap.info/#zoom...TFFFFFFFFF
(compare this to Europe ...)

And on the other side of the blue mountains, humidity is not an issue. Conversely, droughts are.