It was just an example of different ad structures for different length videos, not anything close to a final proposal. I mostly watch very long form content (>30mins), so my example is probably not the best for shorter vids.

This seems like a crazy amount of ads to me. On live TV, I wouldn't expect more than one ad break every 15 minutes of broadcast, with fewer on things like feature films

It's sadly not when compared to live TV. The thing to remember about live TV ad breaks is that commercials are longer, and so are the breaks. On TV, each commercial is closer to 30 seconds, and the breaks are closer to 3 minutes. ~12 mins/hour (4x 3 mins breaks per hour) for TV vs ~3 mins/hour (6x 30s breaks) for my example. Even if youtube doubled up and put 2x 30s ad breaks in a 10 mins vid, that's still 6min/hour of ads.

Front-loading the ads makes sense for shorter vids. I chose the middle because youtube seems to want to put them during the video, even on the shorter vids. I chose 30 seconds based on ads being 2x 15 seconds long each, which is what I normally see while watching Youtube without adblockers. Maybe prioritize shorter ads in shorter vids? So, 2x 5 second ads instead of the 2x 15 second ads.