Team Depth

Hey! 

I am quite new to the game, but I am absolutely invested in it. Although, I have some unanswered questions. First of all how important is team depth? I built a really great squad, 93+ players in every single second or third position even… Then I realised in few of my plays the secondary Free safety starts or ofc every DT is on the field not just the Dt1 and Dt2.

After that I have watched some videos on Theme teams and stuff and I built myself one. But to get 20/20 I needed some weaker players in some secondary positions…

What positions are essential? Do I need a second left and right tackle? Edge? Do I need all 5great Corner Backs? Where can I put the ‘fillers’? 

Cheers!