While it’s not an immediate solution, Framework laptops are way ahead of the curve in terms of open sourcing their firmware, and being open and Linux-friendly in general. The Framework 16 should be out by the end of the year and will support an external gpu.
I would go with the regular desktop version of Ubuntu because while laptops work just fine as personal/small-scale servers, any idiosyncracies tend to be around stuff like sleeping, power management, what happens when you close the lid etc. Whether you’ll encounter any of that depends in part on the laptop make and model, but Ubuntu Desktop is probably the most polished distro out there in terms of handling those things. Edit: though maybe I’m wrong and Ubuntu Server would have better defaults around those sorts of things? Never tried running it on a laptop before.