Perfect. Yes, so you need to add the manual static default route pointing to your SD-WAN virtual link (zone). The member interfaces inside the zone will still maintain all of their respective dynamic routes (including defaults, if any). These routes will and can be used in the SD-WAN link selection criteria on top of your SD-WAN rules and SLA matching (i.e. FGT will look to see which interface has best route to destination still).
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.