I am at a lot lower level than you ( 13 ). I have 5 teams who all use the same formation and tactics. Now as a manager I am still in the learning phase so some season results have been affected as I tinker and learn. Now this season it seems to be coming together properly as far as I can tell. My 5 teams have just finished the season for 2 league champions ( both lost just 2 matches and were around +60 for goals) , 2 finished runners up ( having lost 3 matches for 1 team and 2 matches for the other both also +55-60 for goals ) and 1 team finished third but only 3 points from league champions ( 3 losses and + 50 goals) but won the supercup.
I use 2 different but similar formations , one a little more defensive than the other. I use a 3-1-3-1-2 system ( DL.DC.DR -DMC-ML MC MR- AMC -ST ST) with a more attacking option of replacing the ML and MR with AML AMR. I am finding my defensive setup is reasonably hard to score against , average over the 5 teams this season is just 0.65 goals a match. I find this system is able to break down teams with back 4 and no DMC almost at will , my AM's are all decently quick ( not mutant fast ) with my DMC and MC both skilled playmakers I can exploit that space between the back 4 and midfield very effectively.
I did experiment with 3 DC's but found anyone who plays wide can get in behind you and use the time and space out wide to create havoc. Since I reverted back to DL.DC.DR that threat has been limited.

I am as I say only level 13 so most probably my system has not been truly tested against mutant type AM's except with the possibility of a few in ASSO's where the 5 teams are in GOLD1 but even there the teams are highly competitive and we will finish this season ( our 1st in GOLD1) inside the top 20 and pretty comfortable at , I would guess , about 1275 points ( currently 8 - 8 in the last final for this season.
I am going to experiment again next season with just 1 ST with maybe an extra MC or AMC , I dont know how that will go but you got to try things to find out.

It is likely managers with more experience than I will have a different outlook on this and maybe they can see the pitfalls in what I am doing but all I can tell you is what my limited experience has shown me. Good luck with whatever you decide to do.