Just go with the GPD Win Max.
In my opinion, it's just the better System, since you get a full keyboard, Game Controls and a reasonable resolution for good performance.
It's a real laptop with game controls.
There is one killer argument in favor of the WIN 3.
It fit in here: https://pyra-handheld.com/boards/threads/pre-gamescom-reveal-a-vision-became-reality.99165/
Win MAX would need a complete new design.
ONEXPLAYER is too big as well.
That said, I have already bought a second suitcase and foam to make a GPD (or whatever) + eGPU build.
I don't own an eGPU yet, and I don't know if I'll ever get one but it's something I consider.
Buying a lighter notebook and make the GPD my powerful gaming device.
For now I have a gaming notebook + Switch.
When my Notebook becomes too old, we'll see.