My wife and I bought 2 x Tab A6 (SM-T580) in 2018 - they're very good. EXCEPT that the RECENTS KEY and the BACK KEY are INVISIBLE to us in most lights. We are both 80.
Solution - this is what I just did - it may help others.
I cut 2 small squares from a roll of white insulating tape - each about 2mm square. And stuck each halfway between a button and the bottom edge. I have no problem finding these buttons in the dark now!
Hope you enjoyed this bit of low tech.