Just giving my suggestion here to eliminate tankers.
What if every team that is fail to promote the league will have their star players (players with quality above 100%) automatically reduced their quality to maximum 100% by system. Imagine personal trainer slider to increase quality but this is the opposite.
I think by having this 100% quality cap (or whatever the number will be) surely will disencouraged tankers since the effort to build mutant team would be a lot harder.