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

We convert the user's mailbox to a shared mailbox as part of the offboarding process. Assigning read permissions is only a few clicks away. Delegated/full access over a mailbox doesn't need to be via a PST file, and automaps by default in Outlook. 

As for retention, we use a 3rd party solution with infinite retention using journaling. 
The shared mailbox is removed after 6 months when a task is kicked off to delete the associated user.
I understand you may want to prevent modifications to the mailbox for legal reasons, in which case ediscovery or pst export is your only option. You could still probably automate some of it and spit out a PST (using powerapps and powershell scripting) of offboarded users to Azure files and send the manager (using the AD property) a link they can use to read the pst file.

PS: OneDrive's 'create link to files' is flaky, however, and that's a poor solution that needs to be fixed.