The way I see it, we'll spend a big part of this season tying up the Star/Marco/Jackie arc with some "Moon deals with Eclipsa-related shit" episodes in between. So by the end of season 3 Marco and Jackie would be broken up (or not and never will be) and the season will probably end by showing Eclipsa's escaped/released.
Early season 4 starts by picking up the romance arc and we'll probably have confirmation on Starco happening/not happening by the end of the first half of season 4 (that is if Jarco's already broken up by the end of s3 at this point), but it will also overlap with Eclipsa reviving Toffee.
So by the end of season 4, Star would be facing 2 villains. One that's got the scheming, planning power with the protection of the only mewman capable of matching her magic powers, so she'll have to learn how to fight smarter giving her some form of character development.
That way Eclipsa and Toffee would only appear sparingly to set the climaxes of each season, reducing the cost of paying Hall and Bianco but it still keeps the narrative throughout the series well balanced for 4 seasons.