Sure you can use the one Ethernet port as WAN but rumour has it that it has crappy wifi, it might not even be able to do mesh.
I do not have one and maybe I am wrong but it is something to research before you set out on this endeavour 
I actually use a Pi4 with just the single Ethernet port as my road warrior VPN. It is very fast, and the single port does not cause any issues at all (wifi is disabled). In fact, I have measured between 300-500Mbps WG throughput when away from the house, but I think that the limits have been related to the speed of the remote connection (my home ISP is 10Gbps symmetric; the Pi has gigabit Ethernet).
In my case, the Ethernet port is setup on the lan and my main router is configured with a static route for the WG subnet, but it would be just as easy to use masquerading on the lan zone and WG would route without issues. (One could just as easily use the Ethernet port as a wan port if the upstream network isn't fully trusted, the principles are still completely the same, it's just a matter of the administrative access). The only other thing that is necessary is the port forwarding from the internet through the upstream router to your Pi.
As long as you can connect the Pi via Ethernet to the network, it's an excellent option. But if you need to use wifi, the Pi isn't going to do well (although it might be good enough -- you can get by with ~10-25Mbps for streaming services).
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