you can do this in SharePoint today (i.e. put all members into Read Only permission bucket)Quinn commented
Try this instead = B.S. In a SharePoint Communication site you can have visitors but with a SharePoint Team site associated with an Office 365 group you cannot, it's either full member, owner or guest (external to the org). It would be great to have visitor (internal to the org and read-only) access for all things Office 365 Group including the SharePoint Team site and associated Planner board.