Want all of my iPhotos libraries combined to 1

Is the system version given below your question still the system version you are currently using? MacBook Pro 15″, macOS 10.15?

I am asking, because you may want to upgrade to a more recent system version, if possible. On macOS 12 Monterey re later you can directly import photos from one library into another library, as described here: Import photos from another library in Photos on Mac – Apple Support (UK)

This will only import the photo and videos, not the albums or folders, however. So you should import the smaller libraries into the largest libraries, to preserve the structure of the largest library.

As you "believe" that all your photos are in iCloud, you could get the best, lossless merge by making all your Photos Libraries your iCloud Photos Library in turn. Only one library at a time can sync with iCloud. If you make a different library your iCloud Photos Library, this library will be merged into the existing iCloud Photos Library, avoiding duplicates, and the merged library will sync back to devices. This will give the best results, merging the albums, folders, keywords, faces as well, but it will take a lot of patience for large libraries.

If your system is still older than Monterey or you want to merge the libraries in iCloud, you may want to have a look at this user tip: Notes on Merging Photos Libraries - Apple Community

iCloud Photos does not remove your Photos Library from your Mac. You need al local library on all your devices as a handle to work with your photos and videos in iCloud. The iCloud Library will be mirrored on all your devices.

You can save some storage locally on your devices by turning on "Optimize Storage" for iCloud. Then not all original image files will be mirrored locally on your Mac, but this will make it harder to make backups of your photos. Time Machine can only backup items, that are stored locally on your Mac. I am avoiding the "Optimize" feature, because I want an easy and automatic backup of my photos and videos.

If you can't upgrade to Monterey and beyond, then you may want to check out PowerPhotos  ($40) which has a libraries merge feature. The app is known to be stable and safe for Photos, and it has lots of other features.

If you have a lot of organizational effort into those other libraries that you want to preserve, i.e. like albums, keywords, comments, etc., you might want to consider the paid heron of PowerPhotos. It can import albums, smart albums (as album) and folders whereas Photos can't. Here is a comparison between PowerPhotos and Photos in regards to what they can and can't import:

PowerPhotos: Photos:

Albums -----------

Smart Albums (as regular albums) -----------

Folders -----------

Captions Captions

Titles Titles

Original images Original images

Edited images Edited images

Locations Locations

Favorites Favorites

Can detect and exclude Duplicates detected after import

duplicates (or not) during import

Neither method included projects.

Both methods could import older iPhoto libraries into a Photos library when the iPhoto library couldn't be migrated into a Photos library.

Just some food for thought.