IP SLA for redundant internet route

Afternoon networkers,

I have a few branches that have local internet connections along with their WAN connection. Here is the Topology

I have been trying to figure out how to setup SLA so if the local internet goes down, it will automatically route Internet requests over the WAN connection so users can still reach the Internet.

-Internet IPs are DHCP on the local ASAs -the routing is handled at the 3560 for hosts, default route goes to the ASA

I have dabbled with IP SLA and tracking on the switch, but not sure if I should be using path-echo or icmp-echo nor am I sure what to target. If I target the gateway of the ISP or google dns then the track will come back 'up' when the route is switched to the WAN link, because those are public.

Some of the examples online and in training videos don't have the topology setup as I do, so I am reaching out to this forum for thoughts.

Thanks