I agree about the USB Ethernet dongle. Unless you only require short distance wifi range (eg hotel room temp router) the radio in the pi isn't going to be enough
I built a pi4 router a few years ago, and it's still running great, I recommend the project. But unfortunately I can't find the HOWTO and it was before I started taking good notes. I assume your current router is a phone company supplied modem/router?
My setup is cable modem-->pi router-->switch--> old netgear router in Access point only mode
Being that your router/modem is upstream of router, I'm not sure if you could pass-through the WAN to the pi router, and pass back the LAN to the router/modem for the wifi... maybe someone on here can shed some light?
Looks like the same over on Reddit, it's really sad. For a while I got sooo much excitement following Pine64. I feel like I was just working to make money to buy everything they were developing!