Lock screen ads gone, all the trash apps you hate gone ----- issue remains that Amazon automatically tries to keep your tablet healthy by doing 100% zone replacements that, over time, turn your tablet back into a stock Amazon Fire device.
Note: it does not remove or modify your Google stuff, it just replaces the turned off Amazon stuff with a default turned on current version.
For example when this Amazon lock screen shite comes back up to haunt you, this signals me to simply hook the Windows computer with the Fire Tools on it back up by USB cable and select your own personal change mixture and hit execute. Fast and easy to do, and if you do it 2-3 times during battery life of the Fire Tablet in question it will keep all the Amazon trash at bay.
There is an advantage to some of the Fire stuff, make no mistake about it.
If you have Amazon Prime you get to read a whole lot of free Kindle e-books and a constantly upgraded crop of free movies to watch and music to listen to.
Amount of memory does matter, but if you turn off (forbid the loading of) some of the Amazon crap then the stock device has plenty of memory to handle whatever you do use.
Memory limits do say if you want to play lots of games and Google too, then some of the Amazon trash HAS to go ......
Every generation of Fire gets a bump in processor and memory capability, so this has not been an issue for my household. My wife keeps a spare 8" new in the box for when hers gets wonky on her or the battery gets too puny for words.
...... or she needs a tablet color change to spice things up a bit, you know how it goes. 
I have a Google "new stuff feed" that I am addicted to. I wake up in the middle of the night and check to see what has transpired in my varying world of interests.