When I started my forum I did it because I was passionate about my topic, I did not want to make money.
I had enough cash to run it for a year to see how it went.
I started the same way...as a hobby. Worked great for a few years....donated hosting space, minimal time requirements. Now the site has been fulltime for over 10 years and my hosting costs push $30k/yr.
Consulted with Google in my VERY early days and still have an AdExchange account. I still remember one Google meeting (2006 maybe), I was asked "what are your future monetization goals with Ad Manager?"....my response: "Have Google make as little as possible"

As I explained, and as she already knew, Google serves remnants....the goal is to serve a higher paying ad from another provider. At that time, it was an ugly passback system with frequency caps and other tricks that kinda worked....and I was working with over a dozen networks (many of whom don't even exist anymore). Then I dove into prebid and working that system. Got "fairly" good at it, but it was hours of work every day. And it changed every few months as the system matured, so it was a constant relearning.
Now, the "guest" users see a managed ad setup and "registered" users see my own tightly control GAM-managed ad layout.
Fortunately, my passion for the general forum topic (4wd/offroad) hasn't changed in 30+ years.
I do agree with allowing your content to mature prior to adding any advertising. You need that free-flow of users and participation to launch. It's hard to know when to start the ads tho, but one bit of advice: The "ad load" for registered users should be FAR less than that of guests. I'd start with a max 970x250 responsive navbar ad for all users, then include one in the "quickreply" and just below the first or 2nd post for guests. That minimizes the intrusion for those who are creating your content.