I think you might be preaching to the wrong crowd. Fastmail users (which I am one of) are mainly interested in Fastmail because email (and co) is the only thing they do.

> "No creepy typing predictions" - Fastmail just advertises a missing feature as an advantage

To the privacy conscious, that’s a feature. I and other users pay them enough not to have to harvest, collect, train ml models on, or use our data or metadata in any way, for predications or other pseudo-features.