A lot of the time defending errors are made in midfield.
4-4-2 is the simplest, least complex system and you should be able to get a team to defend in 2 banks of 4.
Also the three centre midfield vs the two centre midfield thing is a bit of a myth. Often it ends up 2 v 2 as very few teams play a flat 3. There are lots of tactics videos on this.
As for full backs, that's pretty much the case at every club for decades. They still have to defend though, just the same as strikers sometimes have to chase back or get behind the ball and wingers have to drop back off the ball.
And there in lies the problem for me. Too many players at this level are either a bit lazy or don't have the engine. To play at the very top you need the technique but you also have. to absolutely work your arse off. That's what I don't see very often at this level. If you could get players at this level to physically work as hard as players at the top level, even without the technique, you'd dominate.