We want to use this for our agency, we don't want all team members being able to access all links etc..

It's very unclear what you mean..

e.g. if we buy tier 5 .. we'd want that across all our workspaces, not per one workspace.

So.. love the idea of have having this.. but doesn't make sense to have to buy tier 5 multiple times to be able to manager multiple client groups ..

Seem like a real shame, and means you don't have real group / user management for agencies, collaboration groups.. if this is correct..