You already know what works: organic forum engagement gives you excellent conversions.
The problem is the hit-or-miss part. That's the actual constraint to solve.
Pick the two or three forums where your ideal customers actively ask questions about the problem your product solves, not where founders hang out.
Spend 20 minutes a day answering those questions thoroughly before mentioning what you build.
This builds trust and relevance. The conversion quality difference you see between paid ads and forums tells you where to focus.
I've also found that forums (in my case private FB groups) are the best source of leads, so I built an app (GTM Intel) to check them frequently and give me a daily list of leads to respond to.
P.S This is very meta / ouroboros but your post is actually one of the ones it found for me today!