I’m in a similar situation in !ultralight@lemmy.world. Getting people engaged is a bit like getting people to dance at a party, many people want to but someone has to break the ice… things I’ve tried is pinning a post asking people to stop by and say hi, also comment on each post so there’s something to latch onto, people won’t click in with 0 comments. You can also try a joke post to break the ice, in the case of SpacePorn you could combine a picture of a galaxy and a swimsuit model and say “Am I doing this right?!?” or something; it’s stupid but hopefully it makes someone laugh.