I’ve been using ARM Debian desktop stably for a long time. I don’t see what the issue is, am I missing something or is this just his hardware choice?

Largely hardware-specific problems, yes, which is typical for ARM in my experience. Although, I would argue that

> It turned out that there was no org.freedesktop.Platform.GL.nvidia in Flatpak repositories for AArch64. And I used both of those tools quite often.

is more on the side of being a software problem... with this particular hardware.