Amazon's Fire series of tablets may provide a lot of value, but the company cut a lot of corners to achieve those price tags. Take for example the Amazon Fire HD 8 - the 2018 edition was launched with Fire OS 6 (based on Android Nougat!), but the tablet has yet to receive a single major OS update. While you can sideload Google Play Services as well as Google Play Store onto the device and disable a bunch of software-imposed limitations, Amazon's custom Android software falls short in numerous instances that are simply not possible to patch. Luckily, you can unlock the bootloader of the Fire HD 8 (2018) using an exploit and subsequently install TWRP, which means the Fire OS can be replaced with a close-to-stock version of Android.

Amazon Fire HD 8 XDA Forums

XDA Recognized Developer Kaijones23 has been compiling unofficial LineageOS builds for the 2018's Fire HD 8 for a while, and now he has come up with the first Android 10-based LineageOS 17.1 ROM for this tablet. Keep in mind that the MediaTek MT8163 chip inside the Fire HD 8 is no powerhouse and hardware-backed decoding is currently broken in the ROM, so you might face occasional lags. Despite the SoC being 64-bit capable, the Android OS on this model is running in 32-bit mode. As a result, you have to opt for ARM variants of Google apps packages after flashing the ROM.

According to the developer, the current iteration of LineageOS 17.1 for the tablet should be treated as beta builds. While most of the features are working, the ROM still needs to be polished a bit before it's recommended for use as a daily driver. The kernel source code and the device tree used to build this ROM are hosted on GitHub so other developers can submit patches and fix existing bugs. Nevertheless, if you still want to try out Android 10 on the 2018 Fire HD 8, you can head over to the XDA thread linked below to find the installation instructions and download links.

Unofficial LineageOS 17.1 for the 2018 Amazon Fire HD 8 — XDA Thread