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For me an SBC is really a PCB with components on it à la RPI, I haven’t see a lot of x86 like that except industrial one and they are $$$.
Have you thought of a beelink mini PC? This one is $149… https://www.amazon.com/Beelink-Lake-N150-Upgraded-Computer-Business/dp/B0F3C6R47G
Have you tried MX Linux? It is based on Debian, they have a distro for RPI, and they have no systemd
I am using Linux since the 90s, used Ubuntu a lot at one time, then started using MX linux, 16.1 iirc was my first install. Then I continue to use it, I have always like Xfce (coming from mwm and such), and no systemd, no snap, no flatpak etc. MX is very stable, use the latest package in .deb format. I am using it for almost 10 years now, 24/7, I am using it as my work PC too.
Title bullshit, we have multicore machine for years, I can guarantee you this had about no impact else people running Xeon or Threadripper would have saw it at first try 15 years ago.
This looks like to have an impact on the scheduler but not on how many cores are used.
insert xkcd picture about standard