Any advice on a self service way of having managers access mailbox from terminated employees?

Greetings,

I'm looking for some advice on a challenge we are facing with accessing mailboxes from terminated users. Currently, we have some managers who need access to terminated employees' emails for valid business reasons, and our current process involves exporting PSTs from eDiscovery, which can be time-consuming and cumbersome. Moreover, once we pass the PST to the requestor, we lose control of it, which is not ideal because it's not subject to retention policies.

We've considered creating a shared mailbox, importing the PST there, and giving access to the requestor, but that takes too long and involves too many parties in the process (exporting the PST, legal team, creating the shared folder, X team, giving access to said shared mailbox, eventually removing it, Y team, etc.).

I would like to know if there is a self-service way for approved employees to access mailboxes from terminated users (users that no longer exist in Active Directory and are only available in eDiscovery). Any insights or advice you can provide would be greatly appreciated.

Thank you in advance for your help.