As a newby newcomer I come up with new ideas
I suggest some simple rules which may help against tankers.
If you won Champions League or Super League - you promoted.
If you won any of the season club event, like King of the Hill, Road to Rio, Top Scorer Challenge - you promoted
Not sure here about Top scorer challenge, its kinda players challenge, not team, but still - I would use it also.
If you were part of Asso, which got promoted to next division - you promoted.
All of that you can represent as rewards for great performance for regular managers.
I have three tankers in my league (level 4). Two of them have King of the Hill emblem.
One of them decided that he already secured his staying in a league and started playing with strong squad, messing up results (probably to gain some skills to his players)