I don't think I can really help much, but there are some comments you have made that
I am not sure what you mean, or how to interpret them, also particularly this:
3) debian testing + firmware-linux-nonfree package -> same result
Please show us your /etc/apt/sources.list file, so we can see what you have in it. Use code boxes, please.
apparently Mint and Debian Buster uses similar versions.
Bios configuration must be right because Mint works with the default settings.
But, in spite of "similar", and to some extent Linux Mint is based on Debian, they
are NOT the same, by a long shot, so yes it is very much possible something works
on Linux mint, but not Debian, Debian is more reliable though, Debian uses software/firmware , etc. that has been tested and used for sometime, and only software that is determined stable goes into the repos, where as Linux Mint ofteh uses the newest, shiniest versions, often these versions have not been thoroughly tested and have bugs that show up later,..personally, and I learned from experience, stay away from Linux Mint, it is not reliable, ..like Debian is.
The other comment:
5) linux mint 19.3 (5.0.0 kernel) with non-free software option -> working well, although vertical scrolling is not as smooth as, for example, in windows 10 SO.
Again, Debian is not Linux Mint, nor windows 10, both might seem better to you, but also Linux, is not windows 10,...if you feel that Windows 10, and Linux mint are better, that is fine, just use them... Comparing Debian to those is pointless, because Debian is not either one, and will not perform the same, trying to make Debian like windows 10 and linux mint, is pointless, a waste of time and beyond the scope of this forum.
Obviously my goal is to make it work on Debian. It seems to be posible for APUs with Vega 8 graphics (AMD Ryzen 3500U, 2200G and others):
I am sure it is possible, and this is a admirable goal, way beyond my skills (very limited), however, it might be to your advantage to use the mail lists, and work directly with other developers that also have the same goal, ....
It might be of interest, and worth the trouble to read:
https://wiki.debian.org/DontBreakDebian
Pay particular attention to:
Don't suffer from Shiny New Stuff Syndrome
The reason that Debian Stable is so reliable is because software is extensively tested and bug-fixed before being included. This means that the most recent version of software is often not available in the Stable repositories. But it doesn't mean that the software is too old to be useful!
Before attempting to install the newest version of some software from somewhere other than the Debian Stable repositories, here are some things to keep in mind:
Debian backports security fixes and new features, judging software by comparing the version number of the Debian package to the upstream version number does not take this into account.
The latest version of the software you're trying to use might also have new bugs.
Installing software from places other than official Debian repositories are not covered by Debian' Security team.
With that said, welcome to the forum, we do have a member here, "stevepusser", he also does a lot of back port work, he might even have something that will work for you, I don't know, you can search on that, or just wait, he is likely to respond hereif he knows something is available...good luck and have a good day
