What about subtitles & closed captions? I see in one of your comment replies that you’ve uploaded transcripts of your videos but looks like Youtube has 1 spot for transcripts and another for subtitles/CC. Have you tested both transcripts and subtitles/CC? Also, I know the difference between subtitles and CC but there doesn’t appear to be anyone that’s tested once vs the other. Might be something to test.
One of my favorite ways to give content a boost is to run ads on Facebook targeting people with interests that are relevant to the content. It’s fairly low cost since you are offering a free piece of content. By targeting people with relevant interests to your content, you drive the right people to the content and into the top of your funnel. And if your content resonates with them, they’ll share, link, and engage with the content in ways that will help Google see its value.