HP Chromebook G1 goes up to 16GB with the M7 model. I have the M3 one with 4GB RAM and it works well for me with either ChromeOS + crouton or plain Ubuntu 18.04. It should get Linux app support too, it's a fairly recent device.

EDIT: looks like you can get the M7 for <600$ https://www.amazon.com/gp/product/B01M29N9BR/ref=as_li_qf_sp...