SmartSuite needs an additional level of permissions in the advanced permissions selector that would restrict users from seeing any content they are not assigned to. This "Assignee" permission would additionally restrict the user from adding new records, as well as deleting records they are assigned to.

An option to allow the user to create new records would be a bonus - essentially a variant of the existing Contributor role that would allow users to create or edit records they are assigned to, but not view any other users' content.