For those needing to deal with customers/clients/internal teams with Google Workspace/Outlook and wanting JMAP-style (though not JMAP) modern JSON APIs, Nylas might be a viable option: https://www.nylas.com/
Nylas pricing has gotten better recently, but is still quite high though - at $1.50/connected account/month at scale, it's likely material to your per-user margin if it's part of your SaaS offering.
But if you have a use case where this is a no-brainer (like capturing/analyzing/building custom real-time UI around your internal sales team's emails) then it's remarkably powerful.