How Manage Large Photos Library?

You can have multiple libraries. Hold the option (or alt) key when launching Photos and in the resulting menu you can create or choose between different libraries.

The "weight" of a library (in gigs) is not that important. Using a back up utility you can set the library to only back up the changes, rather than the entire 130 gigs every time.

Also there are a lot of alternatives to Photos, which might suit your needs better, depending on those needs and your budget.

See this thread for merging two photo libraries How to restore photolibrary from Icloud,

A second method is export photos from system photo library to a desired location say download folder in finder , then connect an external hard drive to the Mac then transfer the photos from download folders into the external hard drive , and all your photo data is saved .See this article Move your Photos library to save space on your Mac - Apple Support

If you desire how many selected photos see this article Select and deselect photos in Photos on Mac - Apple Support are to be deleted in system photo library folder , you should select them and delete to recently deleted folder see this article Delete photos or recover deleted ones in Photos on Mac - Apple Support ( its your choice to delete them permanently immediately or after 30 days the photos will be removed from the iCloud server ) .

In this way the complete photo data is still on external hard drive , but the space is reduced in photo library .

See this article to export photos Export photos, videos, and slideshows from Photos on Mac - Apple Support

The export/hard drive/all that route, is old hat. Minimizing a library to current needs is basically what I was looking for.

I would simply copy your large library to an external drive as an archive of your current set of photos. Then delete from your current library on your internal drive all photos that you do not want in your daily working set of photos.

The library size is a tradeoff between two drawbacks.

1. If we split the library into smaller units, we can no longer create smart albums across all possibly related photos.
2. If the library gets too large, it will take too long to copy or backup or repair or to sync with iCloud Photo Library.

For me is splitting libraries much more inconvenient and restrictive than having to deal with large libraries for the daily backups. As long as the photos are related in some way and I may want to use them together in project, I am keeping them in the same library. But I do not allow my archive libraries to get larger than half the size of the drive. Otherwise it may not be possible to repair them without moving them to a larger drive.

My SSD primary drive is 240GB. My photos alone is 130GB. That's why it's not on that drive.

I have been talking about my main archive photos libraries on the external drives. The drives I am currently using can hold 3TB, so I would not let my archive library grow beyond 1.5 TB in size. But it still a bit smaller than 1TB, even after forty years of taking digital photos and scanning old films.

No sense in having photos on a primary drive.

Then we are using our Macs very differently. When I am working with Photos I am primarily using my portable Macs. And having external drives connected to a portable Mac while commuting or traveling is cumbersome. So I am keeping a smaller Photos library (currently 250GB) on the internal 1TB SSD. This library is holding all recently imported photos that still need editing, plus all older favorite photos that I might need continual access to. And I am syncing this library with iCloud Photos across all my Macs. This way I have three Time Machine backups of this library included in the Time Machine backups of my Macs at different sites. I am only plugging in the external archive drives when working with my iMac.

An internal drive with only 240GB would never do for me, because the system sound library I am using with GarageBand and Logic Pro X alone needs 90GB, and I like to have my iTunes Library and my iBooks also on the system drive.

Just remember that a Photos library can't run from a NAS. It needs to be on a drive formatted Mac OS Extended (Journaled), and directly connected, not networked.