I moved from gsuite to fastmail about 6 months ago I'm pretty wary of any "automatic import from $previous_service" tools (having been bitten in the past) so

I moved from gsuite to fastmail about 6 months ago I'm pretty wary of any "automatic import from $previous_service" tools (having been bitten in the past) so I ran the fastmail importer against my user's accounts, then later reconciled this against the gmail/fastmail APIs with my own code ... it was completely byte-for-byte perfect (other than "muted" conversations, which weren't imported at all, that I dealt with separately) I was very impressed indeed