I finally got this up and running and have played a little bit of Pokemon Yellow Legacy. It's quite a good experience. What isn't a good experience is adding games to the SD card. I have a nice Linux laptop, so it's fairly easy for me to put in the SD card and drop files in. What's even easier is using Total Commander on my android phone to send new games and save files over wifi. Using the LAN plugin for Total Commander and enabling SMB makes transfers very easy.

In the advanced setting menus on the device, enable windows networking under Toggle Windows Network. Go to the Wi-Fi settings under RetroPie Options, connect to your network, and then reboot. Go back to the Wi-Fi settings and note the IP address at the top of the screen. Go to Total Commander, choose the LAN option towards the bottom, then Add New Server. Name it whatever you'd like, then continue to add the IP address, username, and password. The default username is pi and the default password is raspberry.

If all has gone well, you'll be able to connect to the device and move files into the rooms folders from an Android device. There is a bit of a learning curve to Total Commander, so see the documentation if you need to.

I find this method is plenty fast, and is so much more convenient than putting the SD card into another device.

The game save files are located in the folder with the rom files, so they're easy to move around as well.

I highly recommend setting this up. I'd love to find a way to automatically back up the save files and roma to a Google drive, but I'm not sure I'll get into that. For now it's good enough to copy the files to my phone and then upload them to my drive as needed.

I hope this helps someone.