I don't have any good advice, so I will just list some unorthodox ways to discover new videos:
- Use filmot.com. It allows you to search for channels by keyword among other things.
- Search for specific topics on YouTube by putting the search term into double quotes, sort by popularity, and scroll down. I found lots of hidden gems that way.
- YouTube lets you filter search results for playlists. Since many users put their favorite videos into playlists, this is a good way to find older, unusual, and unlisted videos.
- Search for
site:youtube.com OR site:youtu.beon your favorite subreddit - Search for youtube.com on Lemmy. You can filter by community and add more keywords.
- Install the FreshView browser addon to hide already watched videos on YouTube (probably does not work on mobile).
If your extension consists of just a single JavaScript file you can simply add that to your Violentmonkey script collection. Otherwise, you can unzip the XPI file and load it as a temporary addon (AFAIK temporary addons are removed when you close Firefox).