For track-ip I usually use the DNS servers of the carrier configured on the interface. These typically allow ping and they are a reliable remote system to see if the internet service is up or not.
I don't use default gateway on the ISP because there are many times when this is reachable but the internet access is down.
The remote gateway of the VPN can be a good choice. The only disadvantage there is that the internet may still be working but the remote site may be offline. So this can affect other services using the ISP besides the VPN itself.
I like OSPF for the failover because these will failover when the neighbor relationship is lost on the VPN. Thus the track-ip is really a backup insurance policy.