Best practices to move macOS Mail.app storage to an external drive?
Photos

however the library is too large, and constantly growing, so that is not an option.

I don't understand the reasoning behind this. I assume you have more storage on your external drive than your internal drive. It's the perfect thing to move.  New photos should be added to the external drive. Here is what apple is saying on the subject.
https://support.apple.com/en-us/HT201517

Mail

I'd pick ln command so macOS above the Unix layer will not know that you moved the folders. 

SymbolicLinker is a tiny contextual menu plugin that, once installed, allows any user to create symbolic links to files inside the Finder.

encryption 

Is up to you.  It's harder to recover data that's encrypted.  Your data is less protected.  You have to evaluate what your data is worth out in the wild.  I never take my computer outside my house, so it's as safe as all my other stuff. I have an off site backup too.

backup

Do not forget to backup all you external data.