You call OzRunways and Avplan backups but both are CASA accredited sources for charts, ERSA etc. you pay a comparatively low yearly fee for all updates. The AERA 500, 550 and 660 are toys by comparison. Expensive monthly updates which may not affect Australian operations. Silly Garmin non-aeronautical map display. Small screen.
If you want a decent size display with all the relevant information in a familiar format, get OzRunways or Avplan. If worried about backup, carry some paper and/or pay the few extra dollars for a second subscription on your phone.
OzRunways has excellent support and answers questions on line very quickly. I assume Avplan is similar but haven’t used it.