I make posts to announce my comic updates, and I share WIPs in my stories every now and then. I don't know how boosting works, though.
I try to think of Instagram as having several different ways to create engagement. Update posts get very few likes, but a lot of shares from all of my very generous comic-making mutuals. Humor and relatable stuff gets shared around a lot too, while full-colored illustrations have a better reach in hashtags, and get the most likes. It's probably worth experimenting with, to find what works for you! To me, it seems the best to have it be partially about my comic promotions, and partially about other things, so that people don't get bored 
