You'll note that I said "experiment", not use as your daily/production tablet. With that said, all but the latest generation Fire tablets are rootable, and they are available at the usual places (eBay, Swappa, etc.) in new, used, and refurbished conditions.

There are Lollipop based ROMs for all 2017 and older Fire tablets (Amazon's FireOS is based on Lollipop so it's an easy base to start from) and there is work in progress to port Oreo to the 2017 Fire tablets:

https://forum.xda-developers.com/hd8-hd10/general/rom-androi...