How to merge libraries

The easiest way would be to use iCloud Photo Library. But you will probably need to sign up for more than the free 5GB of cloud storage.

The general idea is to make each of your libraries in turn your iCloud Photo Library. The libraries will merge in iCloud while iCloud tries to resolve conflicts between duplicates. The merged library will sync back to the currently open library.

See: Notes on Merging Photos Libraries

Downloading from iCloud to a folder in the Finder may result in the file creation dates to changed to the day of the download, yes.

But as long as you use Photos forMac to download the iCloud photo library to a Photos Library on your Mac, Photos will show you the correct capture dates in the Info in Photos. The embedded capture dates in the EXIF tags set by the camera should be preserved when you sync with iCloud. And these tags will we saved with the photos, also when you export them from Photos with "File > Export". The file creation dates matter only in the Finder, but the EXIF Capture dates will matter in any Photo application.

For other options to merge libraries see: Notes on Merging Photos Libraries

Power Photos doe not really merge libraries - as documented it merges one version of the photo and you lose edit history and the ability to reve\rt to original - it is not a true merge - only using ICLP is a (almost) true merge

1. Merge the libraries with PowerPhotos: PowerPhotos is a tool to manage Photos Libraries. You can easily browse libraries in turn and move photos between libraries. Merging with PowerPhotos is fast (https://www.fatcatsoftware.com/powerphotos/. It will migrate the metadata and the albums, but you have to decide, if you want to use the originals or the edited versions. So you will either lose the editing work or the high quality originals or create redundancy by merging twice in two passes, once to transfer the originals, and then the edited versions. You will have to add a pass to remove duplicates afterward. See chapter six in the PowerPhotos manual: : https://www.fatcatsoftware.com/powerphotos/Help/merging%20libraries.html
2. Another option would by to keep both libraries separate and use PowerPhotos to browse the libraries and to transfer selected albums as you go.

LN

You do not have to use iCLoud Photo library but it is the only way to merge libraries - you can merge them and make sure download originals is set and then stop using it once the process is complete - no other process comes even close to really merging libraries as described in the article Léonie wrote

LN

that is my recommendation - but of course it is your choice

LN