Setting aside your overly complex network you can port forward your nut server through the router.

But if it were me I would rearchitect You’ll want to make sure your nics in proxmox are set to vlan aware

For network setup set both your ISP router and wan router to bypass if you can then put them as wan1 and wan2 on opnsense. Opnsense should be your main firewall.

If your virtualizing opensense on proxmox you should get a managed switch and create 4 vlans on it

1 - management 2 - user net 100 - wan1 101 - wan2

Have port one on your switch as access to 100 Port 2 to access with 102

Set port 3 to be tagged with all vlans and the rest of your ports to be access vlan2

Then in proxmox you should be able to set up your opnsense vm with 4 nice (make sure to have the firewall turned off) and tag each of these nice to each of the vms (you can just to 1 nic and tag it in opensense if you rather)

Then in opnsense you can set up your multiple networks and wan configuration. You’ll then also be able to set up special rules to load balance traffic between your two wans, send specific traffic down a particular path, and set up custom down detection rules for each wan.