if multimedia is main purpose, then don't think much abt software update..as it is now approx. 1 year old You will not face any issue...as u are getting Tab S6 in a cheaper rate and good performance...don't be confused. I would have bought Tab S6 in your place...bcoz it has everything it needs if you practically think.. You are correct that you will get only one major update now as it has recieved one but for 1 more year you will get proper security updates.
