I do not know about cloudflare's internals, but I believe the 6 connections limit is purely a browser thing. So it should be able to do as many connections as it's able to handle.
In general connections between CDNs/reverse proxies and origin servers don’t get much benefit from HTTP/2 or HTTP/3. CDNs don’t generally care about connection establishment speed or multiplexing to the origin (the main benefits of newer HTTP versions), since they can just create and maintain N long-lived connections if they want to be able to send N concurrent requests. They generally only bother with HTTP/2 to the origin if they need to support gRPC, which has some unusual connection semantics.