Re: SD-WAN and NAT with IP-Pool
I would try like that :
Create another IP Pool with the other public IP address from WAN2 (I don't know if One to One type will break current internet access for WAN2, I would also try also with Overload)
Change outgoing interface from WAN1 to SD-WAN in IPv4 policy
Set Dynamic IP POOL with both IP POOLS created.
MOVE this rule above existing IPv4 internet access rules so this will get hit first .
TEST the rule and test the internet access for the company.
for testing you can try this on cli
diagnose sniffer packet wan2 "tcp and port 25" 4
diagnose sniffer packet "tcp and port 25" 4 - for all interfaces
diagnose sniffer packet wan1 wan2 "tcp and port 25" 4
There is another option for SD-WAN , search on youtube "SD-WAN Rule Improvement: Load Balancing | FortiOS 6.2 " - but you don't have control over which interface traffic goes as this relies more on SLA's. It worth checking.
P.S. - update public DNS records , SPF record to white list all your PUBLIC IP addresses from WAN 1 and WAN2 .