VIEWERS need to still be able to interact with the STREAMER during the ad process, and the STREAMER needs to control how that looks.
If interaction breaks, momentum dies. There are dozens of ways to keep the momentum flowing while an ad is playing-- just look at all of the YouTubers who get sponsored and do skits/product placement during their videos.
Example: If the streamer has the ability to temporarily defer ads and play them back to back, ("collecting" an ad every 15 mins or so, to play 6 at the end of 1.5 hours,) this gives the streamer more control on when their intended content is interrupted-- or, even better, an option to play the ads FIRST in order to unlock SUBSTANTIAL ad-free time, with a timer to your next unlock.
How to not break momentum during those collected ads? Don't mute the stream! The streamer hits "play" on the ads when they are ready, and the streamer can INTERACT with the viewers during the ads. This could mean picture-in-picture, or it could be a visual ad while the streamer is sound-only for commentary on that ad.
Above all else, do not allow ads to be punishing. Make them part of the experience, and the STREAMER controls that experience.