Flint2 & Slate Plus with Dual ISP + VPN Routing Strategy

I’m currently configuring a dual-router setup with a Flint2 running as a VPN server and a Slate Plus as the client. My goal is to leverage T-Mobile 5G as a backup to my primary Xfinity connection. The tricky part is ensuring services that require the Xfinity network, like Xfinity Stream, remain uninterrupted during failover. I’ve been exploring policy-based routing with WireGuard, but I’m hitting a bit of a wall, particularly with my dyscalculia complicating the number-heavy aspects.

So far, direct WiFi connections to the Slate Plus allow for successful streaming, indicating a step in the right direction. However, I need help crafting policies that ensure Xfinity Stream traffic sticks to the Xfinity ISP, even during failovers or when the Flint2’s primary connection switches to ‘tethering’ mode. Any advice, pointers towards resources, or shared experiences with similar setups would be greatly appreciated. Thank you for your time and assistance! I’m eager to get this working smoothly for the dramatic person who wants to (understandably) stream her live TV services without interruption so we can leverage a load balancing scenario if it works successfully instead of just failover.

Any help or pointers is greatly appreciated!!