Turning on FileVault with a symlinked directory
FileVault only encrypts one volume so hard links and firm links are encrypted (but firm links are portals to another volume which isn’t necessarily encrypted). Symbolic links exist on the volume and are encrypted, but their destination files are not encrypted - just the link to the other file is encrypted.

Your plan to move some folders off is just fine, solid technically and you can encrypt both volumes using FileVault and keep everything encrypted at rest - just that there are two different volumes with different keys and you choose if you use the same or different pass phrases for both.