HA with WAN

Configure SD-WAN, that's how you do load balancing with Fortigate:

https://help.fortinet.com/fos60hlp/60/Content/FortiOS/fortigate-networking/SD-WAN/Configuring_basic_SD-WAN.htm

For HA, well, you implement HA as per the below article depending on active/active or active/passive....

https://docs.fortinet.com...1720/high-availability

For the 2 x internal interfaces per firewall we configured BGP routing between the Forti HA cluster and a Cisco 9500X cluster as per your image

I don't think you should need to load balance but you could if you wanted to. I usually think, what matters more, performance or availability. With load balancing you can lose HA if both firewalls are at 60% load and one fails. The other member won't be able to take the full load. With HA one firewall is always doing nothing but should always be able to handle the load when you fail-over. Load balacing makes more sense once you have 3 firewalls IMO.

I believe you often want to have your ISP's connect to a switch which then connects to your firewalls. We use our Core Switches to handle the external connections from our ISP's into our firewalls as well as the internal connections from the firewalls to the rest of our LAN. If you can buy dedicated external switches to go between your ISP's and firewalls that's preferred.