copy photos library fails with "library already exists"

That is a good plan, Tony. Did it succeed?

If the error message appears after copying a large amount of of the library, it might be an internal file in the package that is causing the trouble. Then it helps to copy the library in pieces, as described here:

How to avoid the Error -36 when copying a Photos Library - Apple Community

You could also copy the library with cloning software, like Carbon Copy Cloner.

I don't know if this explains your error but here goes:

"The error -36 can occur, when the volumes are having a different file system format. "How to avoid the Error -36 when copying a Photos Library to another volume":

https://discussions.apple.com/docs/DOC-250002206

Have you used the library on the new volume, for example to test it? This may have created new, additional internal files. The library is no longer in use, right? Otherwise I would restart the Mac, so Photos is definitely no longer running, while you are copying the library.

I have twice had a similar (don't remember the error message) error at the very and of a huge Photos library copy.

At first I suspected it might relate to a tilde "~" (last low-ASCII) character in the library filename but that didn't seem to explain it.

Then I suspected that somehow copying the system library might explain the error when some files were in use.

So in my last copy I made a dummy system library, rebooted, and then successfully copied the actual library.

That's great! 😊