add the ability for ops or players with a specific permission to view other player's inventories and enderchests.
Also, add another permission or let ops edit other player's inventory/echest

I highly recommend InvView for this for this type of thing.

As a general rule, I do not add features to EC if another, more specific mod already implements the feature well.

Related, potentially useful:

• Mods I recommend
• Quilt Server-side Mods (Most, if not all work with Fabric)