Focussing on SEO is definitely a better idea than sharing your videos on 3rd party websites in my opinion, but it all comes down to producing engaging content that people want to watch.
With regards to SEO...apart from the usual attention that needs to be paid to tags, title, and description I've recently finally understood why a good thumbnail is important and why some youtubers spend hours designing theirs.
On the youtube studio beta you can see a funnel that shows how many people have actually seen your thumbnails on screen (Impressions) and how many people could be bothered to click on them as a result of seeing the thumbnail.
In my case - YouTube has shown my thumbnails to 6 million people in the last 28 days, but only 315,000 have clicked on them. That's pretty appalling, and makes me think I absolutely have to do something to make my thumbnails more attractive. And I don't mean nail varnish.
So...in answer to your question about organic growth...if you haven't already done so I would recommend that you take a look at your CTR rate on your channel's "funnel" and spend time on your thumbnails!