I'll replay here my suggestion to try to avoid tanking.
Apply a sort of "relegation clause" to the best players of teams (let's say all players above 120%, or the best five players of the team indipendent from %) that don't promote, and, at the beginning of the new season, the manager can buy them back for tokens (Nordeus prices, so for a 8* player you need to pay thos 1k tokens). I think this would descourage tanking, because it would cost a lot to mantain the team at the same level.