I tested multiple scenarios with port isolation.
It really does not matter if the port is tagged or untagged. If I make the ports of the core switches connecting to access switches as isolated ports, the access switches do not communicate with each other. But they are able to reach the gateway.
In your network too, it would be ideal to only configure the port isolation in the 24p switch which will do the ipforwarding. Hope this helps!
Please share more details about the issue that you are facing as requested above.