I believe if you deployed using arm/bicep then after you delete the VM
with boot disk (while leaving all the rest, like nics) and redeploy the
updated template your VM should keep the same ipconfig and addresses.
Please mind `diagnose traffictest` is designed to test connectivity
between ports but does not provide reliable performance data (due to
multiple reasons, eg. it won't use NP). For proper throughput test use
iperf client and server outside FortiGate ...