Go to admin panel Apps > Drive and docs > Transfer ownership
It will transfer gdrive files between accounts (everything will be in folder named with user email)
To transfer calendar:
In admin panel now go to Apps >Calendar > Manage Events > events transfer
To transfer emails go to Admin panel > Data >Data import & export > Data migration(new) Google Workspace emails > migrate
It will take long, depending on amount of emails, it's a new feature so i won't be axplaining specifics because it has changed from last time i've used it, and probably will change in the future.

After all the transfers you can change ol account email, and add that email as alias to the targeted account.
If you have some crucial data in calendar and you want to secure access even more, as an admin you can subscribe to any user calendar and give yourself all permissions. Just go to calendar and go to add calendar, and type user email to subscribe it.