I would push the envelop for a dirt cheap motherboard, without network or audio chips, only many USB-C high-speed ports (and dongles for those). I guess the power supply or battery pack will be the most expensive component after the APU and before the RAM.
We are talking <500 euros serious gaming mini-PC.
The real way to get a cheaper board is going chipsetless; something knoll3 like this [1], but without the fancy bits. Single 1g ethernet, no ipmi, no oculink, and the price probably comes way down. Although, I do see there's a MSI PRO A620M-E Micro ATX out there for $75, so maybe we can have a chipset after all.
[1] https://www.asrockrack.com/general/productdetail.asp?Model=A...
But for the normal users, yeah, an audio chip and a Gbits network chip would be appropriate (probably routed directly to some APU PCIE pins).
I used to dabble a lot with Linux and its how I got my career in tech actually.
Closest I got to something like that was a gentoo distro, although I think I messed something up because I wasn't able to boot into it after all that setup.
Most of open source SDKs are abominations and code is being literaly brain fucked by ultra-complex syntax languages like c++, rust and similar. I am disgusted, to a point for me, open source software is now hardly less worse than msft or apple trash.
I have open source software only because I can still customize it enough to let me bear with it and it is free as in free beer.
That to say, the amount of insane and broken knowledge you need to properly maintain a fully custom linux distro (very custom) is obscene.
I am trying to find ways out of some of the critical issues for myself, basically trying to move a mountain with a spoon.