Let’s go full guerilla: Plugin that lets you select the first and the last frame of an ad, thus allows to report the beginning and length to a synced database. When that frame is found in the buffer, skip X frames ahead.
This would fit in well with SponsorBlock, which already does the same thing for different parts of videos (eg sponsored segments, intro and outro animations, non music segments in music videos, etc).
I suspect YouTube will find ways around this, like running ads of differing lengths, add random amounts of padding at the start of the video or between ads, etc.