... - my most used computer.

Thank you for the information. Then maybe I'm on the right track.

By the way, do you guys find these fanless machines o.k.? For example something like this?:
https://www.shuttle.eu/de/products/slim/ds10u

That is pretty much what my HP T520 thin clients are, they take an M2 SSD, are silent, & fan free too.

I have been using fan less computers for quite some time now, can't remember when I got my first one, but must be about 5 years ago, maybe more, it's a MSI, about 4" square & 2" high - my most used computer.

a standard desktop?

Yes.

What is your goal, a mobile system or a standard desktop?

... and with a processor like that even GNOME is as snappy as a pool full of alligators

Thank you, I can use that as a guide.
I am using LXQt, but maybe one day it will overtake me and I will want to go to Gnome.
I will then look for a computer with Fifth generation Ryzen 5850U. Maybe I will find something.
By the way, do you guys find these fanless machines o.k.? For example something like this?:
https://www.shuttle.eu/de/products/slim/ds10u

EDIT: and with a processor like that even GNOME is as snappy as a pool full of alligators

... Take a look at the resellers of company pre used computers on Ebay.

Yes, that would also be a possibility. I had a Lenovo ThinkCentre from my employer. I had Devuan on it and I worked with it until my retirement for many years longer than my colleagues who got the same devices at the same time but left Windows on it. I was lucky that my boss said that I can install whatever I want on it, the main thing is that I work.

Towers go quite cheaply, as everyone seem to want small quiet computers these days, (me included).

Take a look at the resellers of company pre used computers on Ebay.

... hot swappable SATA drive bay ...

That sounds good to me. I'll put it on my list. I suspect, however, that we will be expensive ...

Extra storage can always come in external USB drives - what other expansion were you thinking about(?).

I find suitable in my use case hot swappable SATA drive bays. Sure that USB 3.0 is fast enough to backups, but I prefere a real SATA conexion.

... Extra storage can always come in external USB drives - what other expansion were you thinking about(?).

Yes, in the main it will come down to an external USB drive. In the longer term, I'm also thinking about a storage connected to our Fritzbox (but that's not a problem).
By the way, I already ordered a computer with an AMD Ryzen 7-5700G (an extremely cheap offer), but there were problems (suspicion of fraud) and my bank did not make the transfer because of that. For the price I have nowhere else found a computer with this Ryzen.
Of course I want to spend as little as possible.
Only with the processor I have problems. I do not need the fastest, it should consume little power, but it should also be future-proof, I always use my devices 10 years or more.
What I also found are the fanless PCs. I already like them, but I'm wondering if there might be heat problems.
But I also like large cases. You can reach into it, change or expand something.
In any case, the device should be windows-free.
I would prefer to build a computer myself. But as I said I am absolutely not the Hadwarespezialist.

Extra storage can always come in external USB drives - what other expansion were you thinking about(?).