Start on Boot for the VM should take care of that.

My OpnSense is a VM on some n100 mini PC under proxmox. Regular reboots haven’t had a need to attach a monitor in years, or manually hit the proxmox webui for anything like that.

If you skip passing through NICs, virtio can work just fine (1 Gbps NAT throughput on 1G intel nics). For me, this is to have the option of adding a 2nd opnSense or whatever alongside (segmentation or just prepping replacement or stuff like that). I also run small core services (dns) on the miniPC as additional containers or VMs.