You can use a KOReader plugin, https://github.com/stradichenko/audiobook.koplugin, which has work-in-progress support for Media Overlays (the EPUB spec that Storyteller uses for readaloud)!
I'm going to set this one up and try it with kokoro which has the most natural for small size that I've seen. Wonder if the paperwhite 12th Gen can handle it.
And also setup the main repo for when I have the real audio book. Thanks!
My phone went from slightly slower than realtime to very comfortable (also lower latency on first utterance!)