Currently Level 7.
5 league championships (missed season1 when I played only 4 games, and season 4 which I deliberately sandbagged to try and get a better Cup draw under old format),
2 Champions League championships and 1 second place finish in 4 tries.
1 third place cup finish* in 6 tries (Cup Level 9[2], Level 9[3], level 10[4], level 8[4]*, level 8[5], level 9 [6])

Does that count as successful?
I play my best 11 in Cup and CL, play my 11 scrubs in league.
I play defensive /counterattacking (3-6-1 V variations) almost all the time except against a couple particular formations. I usually have the league's top scorer, the most goals and best goal differential by a wide margin.

I don't buy tokens - I spend about 70-80 tokens/season on transfers (picking up 35-45 tokens from videos, offers), always buying top 5* players (as close to 6* as i can find) with multiple positions (DC/DR, AML/ML, etc.), never worry about SAs. Keep 22 or more players on squad. I usually stomp on all-out-attacking teams because they're so predictable and easy to get possession advantage over.

My strategy? massive squad turnover each season, use formations and tactics to 1)gain local advantages onfield (e.g. 4 defenders vs 3 attackers), 2)gain possession advantage (always outnumber opponent at midfield) and 3) exploit weakness (e.g. attack middle with AMC and lone ST vs 4 wide defense). Focus on Cup, then CL, don 't worry much about league.
Your mileage may vary.