I’ve moved most active and new accounts over to Proton, and I’d like to migrate all my old emails too, but before that I’ll need to be able to search mail content from mobile.
This article covers the topic for desktop: https://proton.me/support/search
I hardly ever use search or mail on desktop though - it’s usually on the go I’m looking for information.
When is this coming to mobile apps?
They describe the solution in the link I provided for the desktop functionality for this; it should be fairly similar on iOS except for the limitations to background jobs.