I loved AsusWRT-Merlin back when I used an Asus router.

So, the Kindle OS is really basic.

If you’re able to jailbreak, look at installing KUAL and KOReader and that should change your experience enough to not need an entire new OS.

Kindle Fire was the original name of Amazons Android tablets. They later became just “Fire Tablets” and dropped Kindle from the name.

You’re seeing a lot about flashing the Kindle Fire because they still come without Google apps installed and some users believe that makes them very hamstrung. A custom ROM like LineageOS makes it a “real” Android tablet.