SD-WAN and health checks would be my first choiche. Hoewever if you
don't want to move to SD-WAN you also have the option of using a link
monitor:
https://community.fortinet.com/t5/FortiGate/Technical-Tip-Link-monitor/ta-p/197504
Hi, you can automate the entire process by hosting a list with IPs or
FQDNS on a webserver and add them as a Threat Feed to your
FortiGate:https://docs.fortinet.com/document/fortigate/7.4.5/administration-guide/891236/ip-address-threat-feed
