Anyone who is interested please vote.
I have managed to get Sky Broadband working with a 3rd party DSL modem and Opnsense while managing to keep my existing SR203 router (the latest Sky Q hub). I am more than happy to share this set up with anyone who would be interested. I have attached an image of the high level setup and am preparing a detailed runbook for anyone who might be interested. FYI, The SR203 router (from SKY) is properly connected with a (private) WAN address and has 3 green lights including the internet light, it is acting as a router for the Sky Q MESH.