Building up a solid userbase and traffic levels precipitate monetizing any online community, so keep that in mind, but anyway…
I wouldn’t waste time with AdSense at all, especially at such low traffic levels. You’re looking at maybe making a few cents, if anything. It’s just not worth the eyesore and potential turnoff from current or potential members.
If you’re monetizing with ads at all, go with Buy Sell Ads, do direct sales, or do affiliate ads.
Also, what niche is your forum? Are there any other affiliate products or services you can market besides those on Amazon?
I’m not sure how to do this technically when it comes to Discourse, although selling “Pro” accounts for a reasonable price could be a viable option. You could offer benefits like access to private forums and other stuff, but it all really depends on your audience. You gotta get creative.