The default load balancing mode for the SD-WAN implicit rule is source IP based. This means that traffic will be load balanced evenly between the overlay members, regardless of the member's priority.

To prevent traffic from being load balanced, you can configure the priority of each overlay member to 10. This will make the member ineligible for load balancing.

The other options are not correct. Changing the load balancing mode to source-IP based will still result in traffic being load balanced. Creating a new static route with the internet sdwan-zone only will not affect the load balancing of the overlay interface. Configuring the cost in each overlay member to 10 will also not affect the load balancing, as the cost is only used when the implicit rule cannot find a match for the destination IP address.

https://docs.fortinet.com/document/fortigate/6.4.0/sd-wan-deployment-for-mssps/775385/defining-interface-members