Two choices. Either way you need another drive. And make sure you have a secure backup before doing any mass processing of your libraries.
Split the library, (Copy it, then delete half older photos in one and half newer ones in the other. Then drag one of the libraries to your second drive, and keep the library with the newer photos where it is now.
Or just buy a much bigger drive, drag the library to the new drive and continue to work from there.
In all cases, make sure the external drives are correctly set up for a photos library:
It must be formatted APFS or MacOS extended (journaled) (preferred for spinning drives). If you have not formatted it after purchase it will most likely be ExFat - which is not compatible.
It Must not be (or have been) used for time machine. (A time machine drive can be used if re-formatted)
It must have the 'ignore ownership" checkbox ticked.
It must be directly connected (No ethernet or wifi)
See
https://support.apple.com/en-gb/HT201517