If I could trust that the only thing that would happen with wifi were Pocket, I'd maybe try it now that Pocket is owned by Mozilla who I trust more than most (but not entirely, but at least I trust Mozilla's *intentions* not to be totally misaligned with users like Facebook or Google's intentions can be)…
Anyway, update is that
https://addons.mozilla.org/en-US/firefox/addon/epub/ is mostly working and the dev also responding quickly to some bug reports I entered…