Mini PC for LinuxCNC/CPU Realtime Performance

giaviv wrote:

However I’m a bit limited by the low ram (8GB), low disk space and etc.

8GB should be fine, and how low is "low disk space"? Linux starts complaining only when you get under 1GB of free space, so anything above is fine.

giaviv wrote:

I realize newer models are risky, therefore some of the models I am looking at are far from new generation CPUs (like the Ryzen5 7535U)..

7535U is not "far from new", it is new and still in production, Zen3 with 15W TDP, so it is a laptop CPU and this may be bad for latency, although not always.

giaviv wrote:

Or is CPU not the only potential issue, but also things like the motherboard etc?

CPU's are mostly fine as they can change states and do stuff, mainboards are usually the latency killers depending on quality, BIOS implementation, power saving options, etc, etc, way to many to list here, but mostly bad/weak CPU power stage set to work at the edge and throttling the CPU to save itself from going up in smoke.
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As a general rule, i would stay away from those small PC's, they are cute and stuff, but way to limited thermally, the xxxxU versions are fine, but a 13900 ... oh hell no, it will be limited thermally to something like a good 4 core CPU when under load, and they fail but being limited will prolong their life a lot so...
There were some NUC's that were OK for LinuxCNC way back.
After all this, there is no way to know for sure if any of those will be OK, short of buying and testing.