Growing a community where the content already exists seems easier than coming up with content from scratch, unsurprisingly. I've started 3 Lemmy communities for comics and they keep growing because I post old comics every day. Each one has enough of a backlog that I'm not going to run out any time soon (One of them has ~50 years of backlog ๐ฎ)
I think it also helps that it's not just a simple RSS feed, which already existed for SMBC. Posting old comics along with new comics, but then going through the effort of formatting the posts nicely seems to help a lot. I'm also considering writing a script that automatically handles some of the upstream oddities, like some sites storing pictures as gifs that some apps display as movies instead of images, making it hard to zoom and whatnot.