@madbrain Dual WAN Rocks! Throughout my problems, I never really lost internet. A few things to look for/be mindful.
DNS, you can use the ISP DNS or use some free DNS. I use (System/General setup):
a. 1.1.1.1 - 1dot1dot1dot1.cloudflare-dns.com - WAN1
b. 1.0.0.1 - 1dot1dot1dot1.cloudflare-dns.com - WAN2
c. 208.67.222.222 - resolver1.opendns.com - WAN1
d. 208.67.220.220 - resolver2.opendns.com - WAN2
Important is to make sure that routing doesn't get mixed up.
I would put a monitor IP. Use as monitor IP the Comcast/Sail internet DNS IP to monitor each connection. It makes sure that a bit more than the GW is working...
Have 2 different VPNs. One thru Comcast one thru Sail. Don't mix the things... Yes, it can be done differently but I see an advantage in knowing which VPN I am using.
I would only use IPv4, unless you have a reason to use both... disable IPv6.
and yes, recreating the problem I had is not easy. Only saw it happening twice, in over 2 Years of operations.