There's a lot of factors that go into whether or not a forum can be successful, and I do feel like some of those factors do lay outside of the scope of the forum itself. For one, the most active forums, both now and when we were running them back in the day, are generally both topic focused and/or given a draw from outside the actual forum itself. Like, MalwareUp would've been completely dead from the get had it not been tied to the (at the time) quasi-successful YouTube personalities of Dan and Joe, because the topic focus wasn't kept very strong compared to competitors like the much more active MalwareTips, albeit was still focused enough that it didn't die completely ala Tirade or the 2012 version of Dan's forum. Or nowadays, one of the most successful forums is the LinusTechTips forum which, well obviously, gets most of its draw from outside the forum but is also pretty solidly technology centric to keep it having purpose. Or game forums, like osu! or facepunch back when that was a thing, where the idea was to have a support forum that just happens to have extra sections because why not, but all those extra sections (outside of a catch-all off topic/general section) were related to similar themes as the game(s) in question so it makes sense. My point is a general purpose catch-all forum for the sake of a forum is completely the wrong approach and that's where the concept that forums are dead comes from.
That said, we are also in the app era, and a lot of people nowadays find it easier to find things within an app instead of going to a full on website, but people will go to a website for stuff if they feel they have a reason to. That is what you have to solve for. If you only have a single topic worth talking about, its more likely that people will want a subreddit for it. Your discussion too general and not focused? That's where chat apps took over. Finding the sweet spot is hard, but not impossible. Also having some kind of notable thing worth having noticed from outside the forum itself is pretty important too, or be like all the MyBB people and SEO until your face is blue and hope and pray.
Also, community attitude is important too. If your forum comes off as a place for people rejected from "the mainstream", it'll reek of that and drive people away. Keeping people from being complete shitheads is a necessary evil. I know you try to keep the moderation pretty lax due to how hardcore we were back in the day (I'm incredibly sorry btw) but there is a such thing as too lax. That said, obviously this comes to being important after you already have the community building, but it's important nonetheless.
Those are the main things I can think of at the moment, but I'm sure there's more I'm missing too. And I'm just one bitch so I'm not the end all be all expert on this (or anything for that matter) so hopefully some others have tips to chime in too.
Dying of boredom, I'll try it all...