Should I keep the Airport Extreme network or use the Panoramic Wifi offered?
I am not in US so take with a grain of salt.
If the Panoramic wifi is offered for sale as a combined modem router buy it.. but rental is bad idea. Companies love renting you stuff because it supplies a continuous income stream forevermore.
Single point wifi router is not generally suitable in a large house and you will still need a mesh network of some kind.
A quick google I see cox offers mesh type extenders but frankly you will have no idea if these are good bad or otherwise.. so consider alternatives.
https://www.cox.com/residential/internet/panoramic-whole-house-wifi/pods.html
I think it is better to buy your own modem.. so pick one that cox lists as compatible. Avoid if possible the Arris models as Airport Extreme can have issues with them.
For the rest just use your existing Extreme and Express.. unless your connection exceeds about 350Mbps which is the limit of the Extreme. In that case you might as well go straight to Mesh system.
If you go the Panoramic route then you can still use the Extreme and Express in bridge.. or you can still run the Express by ethernet or wifi in client mode.. they cannot extend wireless in client mode but can do airplay if you want.
At some point you should go mesh. Apple gave up their own airport router line and now sell mesh routers from a few different manufacturers. Better mesh products are coming out all the time.. and prices for average quality ones keep getting better so when you are ready buy what is in your budget and fits your needs.
Nothing replaces the Express for airplay though.. so if you still need airplay to your sound system that does not have inbuilt airplay functionality.. keep the express.. they still work with whatever brand .. with the proviso ethernet is superior by far cf wireless links.