@retropieuser555 Thanks, I researched it a bit, the best action for me is probably to have a dual boot:

microsd with Raspian 64bit and Plex server, ssd via usb with my existing Retropie installation

And have a dual boot option with a choice at startup that defaults to Raspian after 5 seconds.

Any idea how I could set that up?

Best!

RasPlex is a JeOS build around Plex, pretty much like LibreELEC is built around Kodi.

Sorry, didn't make it clear but I was thinking like a Dual Boot:
https://www.google.com/search?q=RasPlex+RetroPie

If you don't want to Dual Boot, maybe the Kodi addon would be best:
https://retropie.org.uk/forum/topic/9673/new-user-wanting-to-use-retropie-and-plex-media-server

https://kodi.tv/plex-add-on-for-kodi

@HomeGrownHeroz Kodi can't act as a Plex Media Server, they are two different things, with some similar features.
Kodi can also act as a media server, although in a different way, all devices needs to have a full Kodi installation. Plex is easier to setup, as a media server and the respective clients.
What you can do in Kodi is using it as a Plex Client, using the addon below, which will be connected to a Plex Media Server, that is what you installed in Raspbian.
https://kodi.tv/plex-add-on-for-kodi