Well there are a few ptp forums working for a long time now, so surely there are methods to make them sustainable over time, for example Beermoneyforum or Forumcoin.
The most common way to make them sustainable seems to be to take in high value ad revenue, which amounts to ads for gambling platforms and scams of various kinds, and pay posters pittances that aren't enough for them to earn a living wage on. This gives the posters dopamine for each pittance they earn, which attracts precisely the online gamblers and people who can be taken in by scammers.
I'm not saying that Beermoneyforum or ForumCoin or this forum employ that exact methodology, but it's probably kind of behind most paid-to-post forums.
Organic forums attract users because of community value (people get emotional support from the community, feel like they are friends and at home) or because of informational value (the site contains valuable information that the users want to ask each other for). Allowing buying and selling between users can increase community value, and providing information about business creates informational value - that's how this site is still online. Organic value is the only real benefit a forum website can truly give to its users. You can't pay people enough to get over the lack of organic value without compromising your ethics, because the user can almost always earn more money doing something other than posting on your forum.
If you want to stay economical, do it yourself. Don’t rely on people for important tasks.
This is bad life advice, and it doesn't work in any money making arena. It is somewhat true in website development, but also not true, for we rely on other human beings and cannot escape doing so. If you go to YouTube, you are relying on the YouTuber for information, if you go to a book, you rely on the author, if you go to college, you're relying on college. Your actions are only as good as your information, and eventually you will let yourself down.
Life is not about becoming self-sufficient, but rather knowing who you can rely upon and who you can't. In the case of managing multiple forums, this quote is especially bad, because you need to delegate tasks and rely on the staff and moderation teams you've hired for each forum in order to survive.