If your bootloader is locked (which is true for the vast majority of 10th Gen units), you can still achieve a "Custom ROM feel" using these high-performance optimization tools:

Most custom ROM guides for the Fire HD 8 apply only to the or older models, which used different exploits. Alternatives for "Extra Quality" Performance

Since you cannot flash a traditional ROM, the "extra quality" experience is achieved using the on a Windows PC. This utility mimics a custom ROM by stripping away Amazon's heavy interface.

Once connected to a PC running Fire Toolbox, you can apply these "extra quality" upgrades: