I’ve been using this a lot lately, and it’s been great after a bit of a learning curve. It even incorporates some of the functionality from the addons and userscripts that I needed for YouTube, like getting rid of clickbait titles/thumbnails and blocking specific channels. Since you never really have a tracking profile when using YouTube this way, it’s very obvious when YouTube is trying to shoe-horn in political channels and clickbait, and you can just continually keep blocking those channels in the recommended section until you get all of them. I’m still missing a way to boost the volume on certain videos that are too quiet for me, though. I use LibRedirect to auto-open YouTube links in FreeTube. FreeTube has occasionally broken because of YouTube API updates, which requires them to figure out the problem and push a new FreeTube release (which could take a day or more), but other than that I’m fairly happy with it.
I’m used to LanguageTool, and at a glance it seems like Harper covers way fewer rules than LanguageTool does. Not sure if this is actually noticeable in practice, but I run my own LanguageTool server and am not too picky about the performance, so I’m not in a rush to move until someone figures out a good way to compare them. LanguageTool’s rules are all open source at least, so it’s only a matter of time before Harper gets anything it might be missing.