Migrate the FortiGate to an Azure F4s_v2:
Azure F-series instances are designed for compute-intensive workloads and provide higher CPU performance compared to D-series instances. By migrating to an F4s_v2 instance, the FortiGate VM can achieve better performance without requiring a license change.
Enable "Accelerated networking" on the Azure network interfaces:
Accelerated networking reduces network latency and jitter by bypassing the virtual switch and enabling direct packet processing in the Azure host's hardware (SR-IOV). This significantly improves network performance and reduces CPU overhead, enhancing the performance of the FortiGate VM.

The debug log indicates that there is no SA proposal chosen, which means the two sides of the VPN tunnel (headquarters and branch office) failed to agree on a matching Phase-1 configuration. Specifically, this happens when the encryption algorithms (or other Phase-1 parameters like hashing, Diffie-Hellman group, or authentication method) do not match.
In this case:
1. The debug shows different proposals being evaluated but none successfully negotiated.
2. The issue lies in the Phase-1 encryption algorithm mismatch, as it is the primary reason for the
"no SA proposal chosen" error during ISAKMP negotiation.