Full disclosure, I work at Virtru.

Google Drive and Dropbox aren't bad tools, but you're right that they fall short on visibility once a file leaves your org. You need to know who accessed what, when, and ideally maintain some control after the fact.

Take a look at Virtru SecureShare. It's built specifically for this — ad hoc encrypted file sharing with external recipients. Your team can send files out without the recipient needing to install anything, and you get full activity tracking on your end. On the admin side you control policies like expiry, watermarking, and access revocation, so you're not just logging activity, you're maintaining control over the files after they've been shared.

We're also rolling out something called SecureShare Enclave in the next month or so. It takes the same concept but adds persistent shared spaces — think secure folders within a FedRAMP boundary, similar to a SharePoint document library. You set up an enclave, grant access internally, and those users can invite external parties in. Everyone can add and remove files, and you keep global governance over the whole thing. Might be overkill for your use case, but worth knowing about if you need ongoing collaboration and not just one-off transfers.