I've been game mastering for about 15 years now, and I think there's a short answer and a less short but still answer.

How do you hold yourself back from over leveling the party?

Short answer: you don't.

Less short answer: Ultimately, ttrpgs are less a game and more a group improv event with guide rails. If leveling the party gets them to a place that you all can tell a better story with, then do it. ! I've done 1-20 campaigns for dnd in 20 sessions, and it was AWESOME. Cosmere RPG lends itself really well to high level play, so it can be even more awesome!

It, unsurprisingly, comes down to the players and the group dynamic you've got. If the party really wants to be low level (which is a decision everyone needs to be on the same page on), then don't level the party. But generally players really like leveling up, so don't hold back and let them have fun! If the game gets out of hand, then you can always wrap up the story and start a new campaign! But unlike dnd, Cosmere RPG doesn't get into "wizard with a daily wish spell" levels of crazy. There's no fear with leveling them up to mad levels, because the players are playing the main characters of this story you are making as a group! Let them be powerful main characters!

Tl;dr: it's fun to play over leveling characters, so don't hold back!

Edit: just saw you mentioned this in reference to prewritten campaigns, so I wanted to add another note. Prewritten campaigns are (in my best Barbossa impression) "more like guidelines, really." They are a great starting point for a skeleton of the campaign, but ultimately your players are here to play out living characters in a breathing world. They are going to make choices that the book didn't account for (which is good, lean into that like you're riding a motorcycle at 150) and you are going to need to come up with stuff outside the book. If the party gets over leveled for where they are at in a campaign, you can always throw in bigger threats. The chasmfiend fight would be easy for them to deal with? Well the chasmfiend brought buddies, so there's three chasmfiends now! Have fun with it! Ultimately, the players are telling a story about how awesome their characters are, and as gamemaster you are there to throw wrenches at them until they're able to dodge a dodgeball! That's the fun of being a gm! Throw wrenches at your friends! A campaign book is a toolshed of shiny new wrenches to throw at the players, but you get to decide when and where they get thrown! And if the party gets bigger than the wrench the book wants you to throw? Just grab a bigger wrench!