For me the “easiest “ retraining is a CM to LIB ,the result will be a player with very good vision ,flair for the position with good positioning perhaps a lack at marking but very efficient in tackling.
speaking about role combinations and tactics I believe a tactical orientation of 5 defending and 5 attacking is the most balanced for middle table teams
swepper keeper 2 inv. FLBs one LIB def, CDcov, HB, 2 Roaming CM 2 wingers and a Advance forward was very effective with Bayereuth SpVgg in mine save now at Budesliga 3.