The way I'm encouraging growth on my subs is to first sub to them using a couple secondary accounts on other instances.
This way posts get visibility across more than just local.
Then I post once or twice a day, so there's a fresh post that people can come across in "all". This is how people do discovery on the fediverse, they see good posts, and find and sub to communities that way.
There's only so much I can do alone, and some of my subs I'm not posting to as diligently as others, not to mention other communities I want to help get discovered, not started by me.
So yeah, more people helping keep up that daily post would help, but going past that, like what c/digitalart was doing at the start, gets real obnoxious with how it saturates "all".