Need Advice Root Samsung Galaxy Tab A SM-T350

[vyper1126]

Hi everyone i am completely new to rooting. I am thinking about rooting my tablet. Need advice. The only reason I want to root it so I can get the SD CARD to be recognized as the internal memory. I keep doing google searches and I still am lost. What exactly is a custom ROM? How do I make a backup just incase I screw up? What are the odds of the root failing? What happens to firmware updates after root. Can someone please explain this to me. I've done something similar with a firestick with kodi. So I assume I can do this just need guidance. Thank You