Benefit of that is that if they have to have a large cast, they could have different episodes alternate on showcasing different characters and their strengths. But, I think they still need to cut down on the amount of characters in general. And you've made a good point about the lines being blurred..
Longer formats allow for more characters. Look at things like Game of Thrones or Walking Dead... they have tons of characters and they work.
I honestly think peak TV structure for having alot of characters is quite literally the Trek TNG model. Episodic with A, B, and sometimes C plots that can utilize different characters within an episode. You don't need to utilize every single character in every single episode, but you can give them all time to shine.
Let's say we expanded Frozen Empire into a modern, 10-episode TV show...
Episode 1 is split between the 1910 Firehouse and the Spenglers doing some initial Ghostbusting to set the tone.
Episode 2 can focus on the fallout from the Ep 1 ghost hunt with Phoebe getting shelved from the team due to Peck and spending some time on her soul searching and what not.
Episode 3 could introduce Melody and give her and Phoebe some more time.
Episode 4 introduces Big Bad, and we start the see the old guys a bit, as well as introducing Nadeem.
Episode 5 can focus a bit more on Trevor's side story dealing with Slimer, while advancing the Phoebe/Melody story.
Episode 6 can have the Ray/Podcast stuff, and the Phoebe/Podcast stuff and finding out more about Melody's backstory.
Episode 7 Granaka starts to really ramp things up, the Melody/Phoebe betrayal happens, Nadeem begins training.
Episode 8 focused on Peck shutting down the Ghostbusters and we get to go more in-depth on the Engineering Division and gearing up for the final battle
Episode 9 starts the battle, expanded to include more ghosts and all that.
Episode 10 is essentially just the last half hour of the movie, but we get to go more in depth with it.