Is there a searchable text version of this resource?

Not sure, but every video shows a huge graph with all of the watches on it, ranked by accuracy. If you just view the latest video you can get an idea of the best watches, or where a watch you might be looking at falls, then you can search for the specific video where he tests it to see specifics.

In the video description click on "Show transcript".

But it seems each video is about a different model; having to search N transcripts still isn't optimal.

He's not written books on the subject. You can setup a search yourself. There's also a blog with easier access than transcripts. https://www.robterhorst.com/post/apple-watch-ultra-2-vs-seri...