Hi Neville, 
many thanks for your suggestions. 
As far as Debian itself is concerned: I was already looking into it but I´m not so sure about it any more in view of the latest update scenario as far as security updates are concerned.
A certain patch (“Out-of-bounds heap read/write vulnerability in VFS module vfs_fruit allows code execution”) is still unavailable for Debian whereas other LInux distros (Ubuntu amongst others) received it quite a time ago.
See:
https://security-tracker.debian.org/tracker/CVE-2021-44142
There is a discussion about it with @Akito here: Ubuntu vs Debian stable (security patches) .
So at present I´d rather not opt for Debian proper. 
But Devuan seems interesting. 
On Devuan – Wikipedia Chimaera (stable) seems supported from the end of 2021 to mid 2026 which translates into a very good support period, I think.
Yet there´s something which makes me wonder:
lists chimaera as “semi-rolling”…
… which is something I actually don´t want. I´m looking for a fixed release-model.
So I´m still not quite sure about Devuan… 
Otherwise it would certianly be a good candidate.
Many thanks again for your help, Neville.
Many greetings.
Rosika