I notice that you mentioned that you wrote a massive crossover fanfic. While this is impressive in wordcount, I think you need to keep in mind that working with established characters means you can skip out on the elements that make a large cast of original characters cause the page count to balloon and the pacing to get snarled up. These elements being: 1. Introducing characters and 2. Giving everyone an arc or role that justifies them being a main character and not a random supporting one who turns up once in a while.
I remember talking about the Avengers movie to a friend who had never read any avengers comics or seen any animated shows, and who hadn't watched the Captain America, Iron Man or Thor movies before watching it, and he found it really frustrating how nobody really gets introduced in that movie; it's just assumed that the viewer knows that Captain America is a straight-laced and honourable soldier from the 1940s and that his powers are having peak human strength, toughness and agility and a special shield that is really strong and bounces off things. Similarly to most people who watched Avengers, people go into fanfic knowing who Sans is, and they're happy so long as he appears enough to satisfy their love of the character, and they don't expect him to change or develop. But if you create an original character, the audience doesn't know who that person is until the writer gives them scenes that establish their personality, problems and goals, and they'll expect the character to be in the story for a reason.
A significant amount of my comic so far, which does have quite a lot of main characters, some more central than others, has been devoted to introducing them all. I've had to get creative with ways of stealthily establishing a character through a funny scene, or working out how to make their introduction feel like a natural part of the story. I have to make scenes work like a checklist of who needs to be where and what readers need to find out about these characters and come up with scenes that do that in a way that feels organic and like we're still moving towards a plot point. When plotting, I constantly have to juggle the logistics of which character is where, and how to get everyone where they need to be, or what order plot events relevent to them happen.
Then of course there's just the extra work involved in drawing it. The more people are in a scene, the more panels where you have to draw three or more people in the same panel and the longer the page takes to draw. Plus you need to keep track of where everyone is standing in relation to each other, try not to break the 180 degree rule but also try to keep it so the person on the left speaks first as much as possible.... It's hard work! 
If you can pull the focus in to a main character and some supporting characters, or say a duo or trio, it'd just generally easier to work with. I know people will ignore this advice, because I definitely did when I was younger like "no! I'm gonna make an ensemble cast series! Like Homestuck!" but... never forget that Homestuck failed to satisfactorily end the character arcs of a huge portion of the cast, was longer than War and Peace and had a format that allowed for a huge text-to-images ratio compared to a format that would work on Tapas. Often a few characters with really strong personalities and powerful character arcs is best, and a comic you can wrap up in a few years means you can start a new one bringing the things you learned into a fresh launch sooner.
