Pulling my fugging hair out over S2S OpenVPN with USG Pro

So a project got dumped on me which involves setting up a site-to-site VPN between two Ubiquiti USG PRO-4. The topology is currently

USG1WAN -> SW1 -> ONT1 -> INTERNET <- ONT2 <- FW <- SW2 <- USG2WAN 

I saw that OpenVPN must be used with the USG if you are behind NAT so I'm trying that with the following configuration

Site 1 Remote Host: Site 2 Public IP Remote Address/Port: Site 2 Public IP/501 Local Address/Port: USG1WAN IP/501 Site 2 Remote Host: Site 1 Public IP Remote Address/Port: Site 1 Public IP/501 Local Address/Port: USG2WAN IP/501 

Even after reading this guide and the Controller User Guide, I still don't really understand what the fields mean and my googling is just finding a lot of client VPN setups and not S2S. Another hangup I realise with Ubiquiti is that I can't even see the status of the tunnel in the controller! Wtf! What IP addresses do I put where?

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Send help please.