I found out about a multi-boot OS solution for raspberry pi. Let's say you want the same device to boot to retropie but also to Raspbian desktop or something else. There are (so far) two solutions that are pretty common. One is Berryboot. The other is Pinn OS. While I don't have all the information on how they work. I may add notes later for whether you can set a default OS. It appears at least with Pinn OS that you hold shift at boot and you can choose which OS to launch to. Another thing I'm unaware is whether there are any problems running upgrades from outside the main parent OS (Pinn OS or Berryboot).