@Zawi deceptively simple to say the least, and it took me a few times to see it in the documentation. I think I did try that before, but the key is that on my Site1 the Outbound NAT did not automatically include the subnet's from Site2, so once I put the Outbound NAT into Hybrid Mode and added the subnets, well things are now working as expected.
I am still using BGP though simply to avoid the static routes, I have a few subnets and am lazy. Couple of things I've learned also is under the Gateway entries, in Advanced you can define the thresholds for latency and packet loss for the gateway to be considered up/down, which is key here. Also, I had the VTI gateway set to disable monitoring, which in my testing also broke the failover, which was another key problem.