WAN - VLANs and MultiWAN

I have a Spitz AX and am curious about how to create a VLAN off the WAN interface and whether that would be listed as a separate WAN connection for MultiWAN or not.

Ideally, I would have 2 providers upstream, both via ethernet. I still want to be able to use WiFi repeater, cellular, and usb modem as backup connections as well when needed. So yes, total of 5 possible sources of internet access.

Since the Spitz AX only has 2 ethernet ports (why oh why?), I figure I can split the wan into two vlans, one for each provider. The only real purpose in this is to load balance across the providers. I prefer to keep the LAN connection as wired if at all possible.

Has anyone done this? I have searched the forums and can piece together some of it, but was hoping maybe someone had done something very similar with similar hardware recently.