As an agency, I want to be able to automatically track the applications I buy on behalf of clients so that my clients can also monitor their software spend automatically, without seeing how much each of my other clients spend

One of the ways to do this would be for users to have "parent" / "master" account, then create sub-accounts for each of their clients.

Then, each application / licence can be assigned to a client and that client can log into their sub-account to see only the spend and applications related to them.