I see what you mean -- content IS king. Being able to take a step back and analyze your work honestly is an important skill to practice. It's also very hard to do - you made the video and spent a lot of time on it and tend to be bias about how good you think it is. If you wait a year or two and rewatch the video, it may come across as cringy and not nearly as good as what you thought of it when you first made it.
Because it's hard to see how "good" your content really is from your own opinion, it's best to look at YouTube analytics for some insight. Retention time is a great statistic because it doesn't lie. You can see exactly when people start to lose interest in your video and perhaps find patterns that you may be able to optimize.
Even still, Watch time is said to be what is currently driving the system. I think YouTube realized that dinging a channel that created a 2-hour documentary with a 30-minute watch time (25% retention) vs. a channel that made a 2 minute video with a 2 minute watch time (100% retention) wouldn't right. I think any channel that can get their users staring at their computer screen for 30 minutes is worth promoting, and I'd imagine YouTube would think something similar.
I'm sure there is some optimal video length vs %retention that will cause YouTube to push your videos out the furthest with the lowest amount of effort. I'm not really interested in finding that balance, but I'm sure there are people out there who are trying to figure it out.
That said, all of the other things, like SEO, thumbnails, etc., are still super important! They are tools to help find an audience. YouTube does it's Recommended video thing, but the tags and thumbnail need to be optimized to have people click on the video through the search results. "YouTube recommended videos" are driven by the algorithm and video performance. This stuff is beyond our control once the video has been published, but we can still control our tags and thumbnails! ^_^