The fact you mention disk passwords leads me to believe that you are familiar with consumer grade encryption, but probably not enterprise grade encryption management. Microsoft offers some good tools for this, so do many of the other security vendors. Most of these tools have complex, rolling recovery keys for whole disk encryption and assigned users are still able to log in with their normal AD, but you can go the route of additional factors or ways of protecting the identities. If you have need of an additional layer item level file encryption goes a very long way of securing email and documents that might be sensitive. That said, you have to equally protect the user accounts that can decrypt the system with tools like Privileged Access Management, Additional login Factors and threat detection tools. I'd suggest hiring an expert for this endeavor, it's pretty specialized and if you get it wrong, you only make it harder to manage, not more secure. Consulting organizations do this all the time, and depending on what you need, it doesn't have to be insanely expensive.