I'll give you guys a "secret" tip.
If you have spare money and you are thinking about buying ads or whatever, there is a way better investment for your coins.
Of course this doesn't work without preparation and without a plan. But if you have the right infrastructure ready, then you can hire from 3rd world countries people who will do "data entry" for you (from sites like fiverr). If you can manage it (communication is a problem) you can build your own wiki, database, gallery, shop or whatever you want to service. The payments are not that exorbitant for you and the guy is happy to have your dollars.
This kind of content is evergreen. Unlike threads which pass away after they leave the 1st page, if you can build a solid database of some sort, people will re-visit your site to check out information. The more expertise information you have, the better. But you need a plan for this to work.
You can also hire people to create threads for you. It will make it look like your forum is active but unless you are lucky this is just temporarily and usually doesn't work and the second you stop paying the people, your forum goes back to its normal dead state (never tried it myself but have seen it fail multiple times). But the "database method" is a constant evergreen content. It will stay there after the paid people leave. But again, you have to supervise it and know what kind of data you want to be collected or entried. Because the other problem is where do you get the data? Again, you need a plan, you need to make a research about your audience and the needs of your audience and service it and find out what you can collect. I can guarantee that this method works since I did it myself. But obviously you will be successful relative to your niche and relative to the size of your target audience. A gaming niche has a target audience of 3 billion people give or take. My niche has 50k-100k at most. Sure, in the gaming niche you also have thousands of thousands of competitors, so it is much harder for you to succeed. In my niche I am the only one right now (doing a non-commercial thing, so it doesn't matter anyway if I have competition or not).
I started in 2018 from zero in a very unpopular niche (like the relevancy to normal people is 0% on average and it is in my native language).
This is how it is right now. Although I was very inactive this year on my own forum, still couldn't finish my projects I planned for years now :S, it is still doing fine. Not happy with user engagement/activity, still trying to figure out a reliable method for that. Ahem, ahem
, it would have been nice if XF wouldn't leave us alone with these problems and do something about it by developing the software, but well we have to keep trying at least.