Welcome! I remember this design popping up on reddit. It's nice to hear that you were able to make some side money off it and pivot into other things to keep the income stream going.
TBH, I wouldn't hesitate to post. Lemmy in general is pretty quiet. I chalk that up to a combination of a fairly small user base and having a ton of communities out of the gate - probably too many for the size of the user base. As is usual, there are plenty of lurkers and if you post content they find interesting they'll come out of the woodwork to comment and upvote. I've managed to find a few communities that match some of my interests and do my best to contribute content, although 3D printing is a bit harder because it takes more effort for me to make what I consider meaningful content. I have considered lowering my standards to see if generates some activity though.
Overall, the vibe here feels a bit like the earlier days of reddit. Lots of techies with a handful of normies thrown in. Since Lemmy is FOSS, the user base does lean more toward the FOSS side of things than reddit, which is somewhat nice in that I'm thinking about FOSS a ton more than I was a year ago.
I'll happily comment! Hopefully that will spark some others. If/when I make something interesting or mess around with my printer I'll be sure to post it here. Meanwhile, [email protected] and a few other gardening and photography related communities are right up my alley.