S1A: Executives have a tight control on the series, but also give it incredible animation thanks to Mercury. Ferguson and Alfonso are forced in to be friends with Marco as part of executive control; the show is a slice of life.
S1B: Mercury is now given responsibility for the Lion King Pride cartoon, dropping this cartoon's animation quality. Executive control has been released due to this trade-off; Toffee appears, and a semi-coherent plot develops culminating in the S1 finale.
S2A: While still partially monster-of-the-week, two plots build up. The first is Ludo's side of the story showing how he's becoming more powerful. The second is Star helping Marco get his dream girl, only to cause herself emotional pain in the process. Both stories cross in the mid-season finale; views are so high Disney allows not only a third season, but also a fourth season halfway through S2.
S2B: Viewership is at an all-time high thanks to Star cucking herself and the revelation Toffee is still alive. Most of the fanbase that didn't survive the S1 hiatus or didn't care to start the show in S2 started here thanks to the mid-season finale, the kiss from Just Friends, or Area's comic.
BfM: Killed Moonfee fags, killed Toffeefags, but still raised interest thanks to Eclipsa's motivations remaining mysterious, Toffee's final line, Ludo, and Star's suicide and #godmode. This is the turning point of the show where everything collapses.
S3A: Filler arc where literally nothing happens between the first four half-episodes and the mid-season. Eclipsa, at first appearing to be a machinist trying to turn everyone against each other so she can get back what she's lost and get revenge at the same time turns out to be genuine, ruining her mystique. Star somehow becomes a crusader of the non-privileged despite this never being in her character prior to S3. S3 is also when both Cotugno and Hirsch affected the series. Mid-season at least builds hype with Meteora.