If I leave, do I really walk away with my data?

Yes, no negotiating: full export in one click — leads, transcripts, scores, appointments — in CSV formats readable anywhere. Appointments placed in your Google Calendar stay in your Google Calendar: they don't vanish with the account. On request, final deletion of histories happens within 30 days, backups included. The only thing you can't take with you is the AI engine and the script configuration — they belong to the service, not to an export.