Best way to clone between FortiGate models

I know you asked "using FortiManager" but have you considered using the FortiConverter service?

https://www.fortinet.com/products/next-generation-firewall/forticonverter

probably FortiConverter is useful. If you want to spend Money for a one time requirement :(

Otherwise you will have to do manually. A restore on the new FGT might work if both have the same port layout. But its not a 100% warranty even if it does. Also both should run the same FortiOS Version for that.

Its only easy if one replaces it with the same model. Then you can simply change the serial in FMG Device DB.