Taking the hand-built FortiGate and Proxmox EVPN fabric and reproducing it entirely in code, one Terraform workspace driving both platforms, with every wall I hit along the way written down so you do not have to hit them too.
Measuring the throughput cost of VXLAN on a small FortiGate with no VXLAN offload, by A/B testing the same VM over an overlay segment versus a plain VLAN, and tracing the ceiling back to the silicon.
Swapping the static VXLAN peer list for a BGP EVPN control plane across a FortiGate and a three node Proxmox cluster, and every place the two platforms tripped me up by disagreeing on what to state versus what to derive.
Standing up a single stretched Layer 2 segment between a FortiGate and a Proxmox host using unicast VXLAN, and proving the data plane before adding any complexity.