I haven't been active on this forum other than asking noob questions (and getting some great answers!). But, I just came across this thread and it caught my interest. I started my own (first) forum last February. I was compelled to create it after the biggest / most popular forum in this particular niche was unceremoniously and suddenly shut down by the owner (for reasons that remain unknown). A few spin-off forums were created by some of the mods and admins from the old forum. All of them were on free platforms and rudimentary in function and appearance.
I decided to make my own, and sparing no expense, which is why I went with Xenforo. No regrets there. My forum is taking off and outperforming the other spin-offs by a wide margin.
First, I'll say that you've gotten great advice already. If I may expand on it a bit...
What you put in is what you get out. I poured energy into my forum by posting topics. I recruited some of the previous bigger participants in the defunct forum. I created a NICE "clubhouse." I made it EASY to use, especially when it comes to posting photos (thanks to Xenforo), which is a big part of our hobby.
While the forum is about a primary subject, I created other sub-forums on various topics that branch out. If your focus is too narrow, it limits engagement. While we may all share the one specific hobby, we likely share OTHER interests. Make it easy to talk about the other interests by including sub-forums to that effect. Talking about one thing can get stale / boring.
Post links to your forum anywhere else you can. Spread the word. Shout it from the rooftops!
Also... I agree with making the forum OPEN. My "rival" has made their forum totally private. And I think that's a HUGE mistake. People want to see what the forum is about before joining. Guests (who can see the content) turn into members. If you want to GROW the forum, make it VISIBLE.
Create a "Home" / intro page (AwesomeName.com) that welcomes visitors and tells them about the forum. Why should they join? What makes it unique? From that page, link to the forum (AwesomeName.com/forums/).
Make sure you get listed by Google. Use Google Search Console to submit your url and your sitemap. My forum now dominates Google search results for this particular niche. So, if I search for "signal and sign collectors forum," does your site show up? Make that happen.
What ultimately makes a forum are the participants. It's the "community." It takes time, of course. But, you'll have to pour "energy" into it to get it going. Prime the pump! It's a LOT of work. Ask me how I know! Ask any of these guys with successful forums.
PS... Yeah, the bright green has GOT to go! It smacks you right in the face when the page opens.