"I have an Xfinity router configured to allow the Airport Extreme"
Just some thoughts.
First, I'm not an expert at networking.
But... seems to me that "the configuration" (on the Xfinity modem/router) that works with the Airport Extreme and Expresses, should continue to work with a mesh system.
I'm going to GUESS that you have the router on the Xfinity set up in "bridge mode"? With NAT turned off (to avoid something called "double NAT")?
Having said that...
Depending on WHICH Xfinity modem/router you have, doesn't Comcast now offer their own "add-on" mesh system? Might be worth looking into.
Be aware that if there IS an Xfinity mesh arrangement that "mates" with the existing "residential gateway" (modem/router), I'm thinking you'll have to take it OUT of bridge mode and restore NAT (if that's currently turned off).