Where is the local copy of your library stored? Is it in your Pictures folder on the system drive or on an external drive?
If the library is on an external drive, you may want to check this with Disk Utility, if it is starting to fail.
Photos may be losing the connection to the system photos library, if it is on an external drive, and this drive is getting dismounted while we are still logged in.
Your library is really large. I have split my library as well.
- My system photos library is holding only my favourites, that I want to access on all devices, and the new photos I am currently working with.
- all other photos and videos are archived on an external drive in yearly libraries and small libraries for each photo shooting travel.
It is easy to split a library, but only, if it has been fully downloaded from iCloud. Then we can simply duplicate the library and delete from the copy all photos that we do not want in this partial library. I am I am using a smart album based on a data range to select the photos to delete from copy.
But before you can split the library you will have to get it downloaded, with all original image files included, not with "optimize storage".