And you mentioning koreader, and a helpful discussion of the topic on koreaders github, helped me finally find an open source reader that does vertical text correctly.
Thorium, sadly not koreader, does Japanese vertical writing correctly and I'm very happy right now.
Newbies, as in creators who aren't good, yet, are going to go whatever platform that they can actually get on.
At the moment that's a self-hosted Peertube or Youtube.