The switch menu is for devices with a dedicated embedded switch. If your system just has „a bunch of NICs“ (which also includes e.g. multi port GbE cards) then resort to bridge VLANs. You can configure those in the device settings.
See the various „DSA“ related VLAN howtos.
To configure a „virtual switch“, declare a new bridge device containing eth2..eth5, then enable bridge VLANs on it. Since every device in OpenWrt needs a logical network using it for it to get brought up by netifd, also declare an unmanaged (or static IP) dummy network interface using the bridge device.
If you don’t have a hardware switch, you will not have a switch menu or any switch functions.
You say you have 6 virtualized ports. I understand the value of two (lan and wan), but what do you need the other ports for? Are they all supposed to be in the same L2 network? If so l, just create a bridge that contains eth2-5
Edit: I see the picture. Those are physical ports, not virtual. Now it makes more sense. A bridge containing all the ports you want to operate together. A bridge is the software equivalent to an unmanaged hardware switch.