> The one area we have had some promising success is in advertising on the site. But the effort it takes to find advertisers and manage them, and my general desire not to flood Exercism with adverts, has meant that I feel this isn't a very sustainable strategy.

To me that begs the obvious question... what about trying some automated advertising like Google Ads? Probably not the most theoretically optimum use of their ad space, but minimal effort is involved.