You should be able to do what you wish through the use of SecuritySuite.
The free version of this comes with SuiteCRM, but to get field-level functionality, you’d have to get the Full version of SecuritySuite: https://www.sugaroutfitters.com/addons/securitysuite
You can set up User Roles & Groups in both the Full and the Free versions.
You could possibly create a group called “External” and assign users to it.
However, The full version of SecuritySuite will allow you to specify which fields a user or group can see on a record.
(.i.e: You can hide custom fields from Users in the group External)