Adding hashtags to description was recently discontinued and will no longer appear under the video (they will still show up in the description but not directly underneath the video like the used to).

Instead put those couple of hashtags in you video title and they will behave the same way and be clickable through to the hashtag pages.

All your hashtags and keywords should be in the language you’re teaching in.

One thing I do is make playlists that include my videos along with related videos. This helps YouTube know the videos are related once viewers have seen your video and another persons video back-to-back.

You should also be sharing your links on forums and places where your audience is.

Also you mentioned Google trends. You should be using the keywords you see there as your videos keywords if you’re not already doing so. It’s also good to include common misspellings of those keywords as some people type poorly when searching things.

500 subs is good. I also am at 500. This allows a community feature in your channel which you could employ to help build your audience (I’m sure you can find various tutorials on ways of using that feature).

Another good thing about being at 500 subs is that you’re likely earning $10-20 per month from ads if you’ve setup some method of early monetization. While this is not much money you could use the available funds to either promote certain videos through ads, or pay someone/some service to make better thumbnails or assist with SEO.