So I think it's worth clarifying your scope in terms of what must be open source and what can be proprietary, especially on the hardware side. On one end you can get any random prebuilt off the street and run LinuxFromScratch, FreeBSD or what have you.
On the other extreme, do you need a CPU, GPU chip, camera module etc. to have released their full specifications and code, do the exact manufacturing blend of the solder you use need to be released?๐คฃ
One site that might be of interest to you is pine64.org, they make open source ARM based phones, boards, laptops and tablets.