I just did something similar last week. My criteria were 1) small form factor like a Chromebook, 2) not actually a Chromebook, 3) could swap out or install an m.2 SSD.
I ended up getting a harddrive-less old Latitude 3190 for 30$ off eBay, put a 256gb SSD in (had it lying around + that’s the max capacity supported I think), and ended up installing fedora KDE. It’s not perfect but for the price it’s amazing