I've found conceptualizing my series as arcs was the best way to go in order to stay on track with what I wanted to do.
In a way, each of my "parts" are mini arcs, so each one has a "main event" that pushes the story forward. But since they're kind of contained on their own, I can do things a little differently with each one (for instance, my Part III is really just one looooooooooooong chapter/event that is numbered differently, and when Part IV picked back up, the chapter numbers continued from II.... confusing? maybe. works for me? yes.)
What I think you're calling "arcs" would be equivalent to what I'm calling in my head "seasons" (same thing, different vocabulary!) I'm still in the first one (which has a name in my head, but I'm thinking of finding a way to integrate it into the title of things later) but the "finale" is coming in a month or so. When its done, it'll probably be equivalent (in word count) to a trad pub book.
I went into my story knowing I didn't want to structure it like a traditional book. Where my first arc/season/book equivalent ends, any traditional reader would be MAD because its almost like the entire first bit is set up for the fall out that's to come, but at the same time, it still satisfies the main first step in the story, which is getting allllllll the characters together in one place (finally) and setting the groundwork for the world/what's to come.
I'm actually really excited to get to the second bit, where everyone has to work together and the stakes start to make themselves more known.....
Anyway... tis me.... (the description is mostly just for season 1, and I reeealllly need to get on fixing that thing....)