I just wonder: if I work with Catalina and FileVault on my (upcoming) MacPRo 7,1 let's say I have a "Projects" SSD where I save data on it, can I just swap it into a 5,1 which uses Mojave and has FileVault off? Can the 5,1 read and write on it?
Turning on Filevault in System Preferences only encrypts the boot drive. By default, it does not encrypt other drives attached to the computer (I'm almost certain about that). I DO know for sure is that you don't need to turn Filevault on in Sys Prefs in order to encrypt or decrypt an external drive.
On one machine, right-click the external SDD drive icon and pick "Encrypt..." Pick an encryption password and ensure you don't lose it! At some point you will be given the chance to save the password in your login keychain so you don't have to enter it in the future but make sure you've got it recorded anyway.
Now you can take that SSD drive to the other machine. When macOS tries to mount it it will prompt for the password (and ask if you want to save it in login keychain). Now the drive is Filevault 2 encrypted but either machine can use it. Again, the System Preferences setting for Filevault has no relevance to this process -- turn it on or off as you wish as it only affects the machine's boot drive (which you will obviously not be sharing between the computers).