Hi Servant of Cats - Thanks again for your advice - I have installed Astropad Luna Display and connected a high performance USB-C to USB-C cable* to both macs with great success.
As you hinted, you have to have a good cable connection to make it work well enough to have the retina display working on the 2nd screen (imac in my case) without too much lag.
That said, there is still a little lag but not enough to impact my experience too much - Noting that I am using my mac as Digital Audio Workstation where the 2nd display (old imac) is being used for passive monitoring and mix console configurations and for that it is generally fine. It helps keep the clutter away from the main screen whilst still highly usable for secondary functions.
That said, I will still invest in a second screen a some point, so its not a perfect substitute but it is a good interim solution that will keep me going a long time until I am ready to poor more money into the studio.
*I have used a Fasgear USB C to USB C3.2 Gen 2x2 Cable 1.8m, 20Gbps 4K Video 100W Charging cable. (Amazon shipped it overnight even down under in Oz :) )