Unfortunately, since 2017, Google has been using a new security chip (CR50) that isnt supported in the upstream Linux kernel. So while you can flash 3rd party firmware and install your own OS, the security chip will throw the device into recovery mode if you ever suspend the computer.

AFAIK, there is no workaround for this - if anyone knows otherwise, please share details. I've got a chromebook that I'd very much like to be able to use something besides ChromeOS on.