You can unlock the bootloader on any Chromebook and install a regular Linux distribution on it.
There's a whole pile of AC700 Chromebooks around, with no upgrade path as far as I know, at least if you don't want to be p4wn3d.
Including AC700: https://www.chromium.org/chromium-os/developer-information-f...
For a while, there were, but they disappeared, and now it's a web of 404s and obsolete documents. Most recommended chrubuntu, but (1) that relies on obsolete ChromeOS components, which introduce security issues (2) someone said that's now dead too.
I'll see if Chromium builds for it. Thanks for the pointer. There's a lot of AC700's out there.