So much info in this one sentence, if you don't mind sharing. could you drop another two sentences on which techniques proved to be not worth investing time and money on?
Highest retention videos: Adwords retention closely follows native traffic retention, so that is a good way to get more views and more watch time on your best (retention) videos.
Highest view count: I found Adwords to be a waster on money. The video is high view because it has been picked up by the algo, so there's really no point putting more money into it, let the algo place it for free.
Lowest view count: Waster of money as well. The videos are lowest view for a reason - they are not that good and engaging (our first 20+ videos will fit this). Money spent on Adwords will bump traffic during the campaign, but traffic falls off to pre-campaign levels after it's finished.
Lowest retention: Once again, the retention is low for a reason. Adwords retention was sometimes higher, sometimes lower than native traffic (within about 10%) so it's not really enough to make a long term difference. No effects after Adwords campaign finished.
I've scaled back on Adwords in the last 4 weeks as there seems to be sufficient traffic from suggested. I'm now monitoring to see native traffic for new videos and get an idea of what videos are popular without promotion.
Prior to that, I experimented with the above strategies, and a few other variations. I mainly focus on promoting videos that resulted in the lowest sub acquisition cost, and the highest earned views.
I've had a quick look at your channel:
You ran Adwords for "Spiderman unboxes peppa Pig mini caravan" which then gave you Earned Views on "Spiderman makes Toy Story Lots..." [No I didn't hack into your account, the second video is in suggested spot #8 so got traffic from the Adwords video]. I recommend to maximize subs you have an Annotation throughout the whole video inviting viewers to subscribe. To encourage more Earned Views setup Cards pointing to your best videos. You may also want to setup Adwords for a playlist and put them all into 1 campaign and select 'maximize for views'. But get the cards and annotations setup first. Once you have data on which videos convert best for subs and earned views, list the factors of those videos and make more of those videos.