Got my Kindle Fire 8 2018 this Monday and got LOS 16.1 on. Performance was great, the only hangup was that there is a known issue for LOS 15, 16, 17 with Netguard not working:
So jumped to LOS 14.1. Aside from a few apps that aren't compatible with this android version, everything works fine. This also worked for other netblockers that use IPtables like AFwall+. I used a kernel manager on both versions to get a bit of a better performance in powersaving/performance profiles. Will also add that I used both without gapps and instead with MicroG/Aurora/F-droid
If you are using this device for a single task/light browsing/as a student, 14.1 should be fine.
Looking forward to bumping to 19.1/18.1 once it's stable