Seeking Advice on Optimising Time Capsule with Optus Cable modem DPQ3925-x

I have a Time Capsule 2TB Model A1409 set up in Bridge Mode with Static address to the Cable router. Configured IPv6 Automatically and IPv6 Mode = Tunnel. The Time Capsule provides our main WiFi network, and is supported by a couple of Airport Express Model A1204 to extend the network throughout the house.

The Cisco Router is set up with DHCP connection mode - the only available option. Firewall is ON. Wireless network is OFF, and Cat6 cable to Time Capsule input port. Typical download speeds from the Optus cable network are in excess of 80 Megabit per second and the WiFi network delivers this easily to Macbook Pro or iMac

PROBLEM 1: Seems to have a limit on the number of connected devices, resulting in important computers or the printer / scanner (WiFi connected HP Office Jet 8620) getting disconnected from the internet intermittently. An apparently online iMac will get the beachball when waiting on a page refresh or loading of Outlook. Is there a way to specify the computers that will never be offloaded from the network?

PROBLEM 2: Transfer speed for movies on a Macbook Pro or iMac to the AppleTV (series 2) is extremely slow. I question whether the TC is sending all our internal WiFi network traffic to the CISCO router thereby slowing down our entire network?

PROBLEM 3: iMac - a high use business computer with excellent signal from the WiFi network - runs very slowly when Time Machine is running a backup across the WiFi network to the Time Capsule.