How to update connected item's status when I need to connect hundred boards?

Hey lovely community,
This is the use case I’m trying to solve to my of my clients:

A law office, they want to manage all the tasks for all their clients on one board.
They want that each client will be able to see the status of HIS tasks ONLY.
They have the Pro plan.

We can’t create a separated board for each client and connect it to the high level board as we have more than 100 clients.
Dashboards aren’t an option as we will need to give the client permission to the high level board.

Any suggestion? We are open to use for solving that.