Would it be possible to run different operating systems by plugging an SSD in with that OS stored?
I’m fairly certain this should work. You can test the exact setup by doing the same with external drives and your current computer. I’m not sure how Windows would react to being installed on an external drive, but any Linux distribution should work perfectly fine.
Storage Expansion Cards are fast enough to install your operating system onto and you can boot from them too. This enables use cases like putting Windows 10, Ubuntu, or a privacy-focused OS like Qubes on a card and booting from it when needed.
From Storage Expansion Cards
Should work just fine with other external drives.