I’ve created a bit of an extension to the ads plugin that may be useful, though right now I can’t release it because it is very intertwined with other custom plugin code. One of the things that it does, which would be super beneficial, is actually calculate the number of ads that could potentially be displayed based on viewport and post length.
I didn’t like the idea of rigidly putting an ad every nth post, so I made it calculate out and insert the ads when the scrolling window did approximately 1-2 page height scrolls. So you pretty much always get an ad after a long post, but you don’t get an ad in between a bunch of small replies.
I may spin it off as an extended plugin, but would love if the Discourse team would implement it into the official plugin if interested.