Keep identity and login mechanism decoupled - plan to support multiple login mechanisms per user (email/password, SAML, OpenID Connect, Google) for a single identity and multiple authentication factors (TOTP, Duo, etc). Be very careful to about what you consider a verified user and how you verify email addresses.
Use TLS even for your database connections. Use encryption at rest. Automate backups and plan to restore or export data for specific customers rather than the whole application.
Use a time series database or event logging system and create an audit trail of everything any privileged user does in your system, any account or permissions changes, destructive operations, etc.