Hi there, have you tried Logic Pro (X) > Sound Library > Relocate Sound Library ? I assume you're talking about relocating from one external drive to another, correct? So, using Logic's built-in SL relocation assistant should be the first step in the process. Just bear in mind that the target drive should be formatted as either HFS+ (MacOS Extended) or APFS and not be used for Time Machine backups. Otherwise, it simply won't be available for SL relocation.
In case the built-in option won't allow to move the SL from one external drive to another, then you'll most likely need to re-download / reinstall / relocate it back on the system drive first, and then use "Relocate Sound Library". Not the most efficient workaround considering the time required for downloading, installation and / or relocation of all those GBs of content, but it should work fine.
I'd agree that what you describe above is an ideal (and, perhaps, also efficient) way to manage sound libraries. And indeed, that's how it works with, say, Ableton or Native Instruments libraries, to name a few. Unfortunately, not with Logic. Personally, it's beyond my comprehension why it's still not overhauled. Most likely, it's cost-cutting and "don't fix what's [technically] not broke" approach on Apple's side to blame.
N.B. You may explore ways to relocate SL on external drive(s) using symbolic links. The major advantage of this approach is not being dependent on SL Manager for relocating Logic's library content. Also it allows to relocate ALL content, including Apple Loops (which, to my knowledge, is currently not possible if one uses "Relocate Sound Library" option). It used to work fine in legacy OSX / macOS and earlier versions of Logic. Not sure about current ones, though. Cheers!