OpenWRT snapshot works flawlessly out of the box; Realtek USB ethernet included. I tried the RPi community build linked above but it has a lot of highly idiosyncratic customization, none of which I needed and some of which was in the way.
With the snapshot, I mirrored all the packages locally to be on the safe side. Yes the kmod packages will remain available for a few months and the rest mostly don't have a sell-by date, but they can develop dependency issues without warning, and you have lots of room on even a small SD card so you might as well just grab everything you might ever want.
OK, looks like you've got a bit of a learning curve in front of you. The snapshot is simply the latest rolling build; for the Pi 4 it's at https://downloads.openwrt.org/snapshots/targets/bcm27xx/bcm2711/
You have a choice of squashfs or ext4, I think ext4 is what most people use, it's certainly more amenable to customization anyway.
The sweet spot for SD card pricing is around 32GB, that's dozens of times more room than you'll ever need for any foreseeable OpenWRT-related purpose, and there's hardly any point in buying anything cheaper: a good quality SanDisk Extreme or Ultra tops out around $10.
Maybe don't worry too much about the kmod issue for now -- but for reference, kmod packages are kernel module packages that are bound to the precise build you're using. When using snapshots in production (i.e. your real-world router not just playing around) it's something to be aware of, but they keep the kmods for each snapshot available for several months; plenty of time to figure out which of several approaches to take for maintainability over time if you decide to keep it.
The snapshots don't have USB drivers Afaik. You need to grab all the kernel modules and install the ones for your particular USB device. Check the kernel logs to see what it detects. Also you will need a USBC to USB a adapter or something the USB c port is power for the pi
Hey all - Ok got the Pi booted up, PPPoE configured and it looks like things are working as they should...
Had to manually create my WAN port for obvious reasons. Do i need to also create the "wan6" port i saw on my Arhcer C7?
I also noticed that there is no switch interface in Luci, is that because the Pi doesn't have multiple LAN ports? I had to instead set eth0 to eth0.35 in the CLI which isnt a big deal, but i was just curious.
The owner seems to be an Openwrt user, since he references openwrt a couple of times. Quite impressive performance for such a cheap device - seems to cap out around ~3Gbps of throughput in total....he posted a video here