For adblocking, You can get Content blockers from the app store that you enable in the settings, Personally I use Adguard Pro.
I've also got an Extension that makes websites Dark mode called 'Noir', You can adjust settings global/per site if they dont look good.
Extensions/Content blockers are limited to Safari (to my knowledge).
Amazon hasn't used the "Kindle Fire" term for a while, They're now just "Amazon" Fire tablets; I would imagine for Simplicity so people don't get one thinking it's just an eInk eReader.
I recently got Islanders (on sale) and It's really relaxing & easy to pickup for breath bursts.