How to merge Photos libraries but retain iPhotos Events?

It would have been best, to merge the libraries in Aperture, while you could still use Aperture.

If you want to retain the albums and folders, you can also merge the libraries in iCloud.

If you make each of your Photos Libraries your iCloud Photos Libraries, one after the other, your libraries will be merged in iCloud, preserving the albums, folders, adjustments and other metadata. The only items, that will not upload to iCloud, will be your projects, like slideshows or books.

merging your libraries in iCloud would be the best version, if you are using iCloud Photos already.

Which system version is installed on your Mac?

• If you are not using iCloud Photos and are running macOS 10.115 Catalina or a newer system version, the merging with PowerPhotos will give you a nearly lossless merge, as well as merging in iCloud. Merging with PowerPhotos will be a bit faster.
• If you are using iCloud Photos, merge with iCloud. PowerPhotos is great, but only the most recent version on macOS 12 Monterey can merge albums from your iCloud Photos Library into other libraries. And only since macOS 15 Monterey you can move the edited versions together with the originals, so they remain one photo.

The best option to merge PHotos libraries is the paid version of PowerPhotos . It will merge libraries including original and edited versions, albums, folders and metadata (keywords, faces, titles, descriptions, etc).

Project are merged as albums with the photos that were in the original project.

The Photos version in Ventura, out sometime by the end of this month its estimated, will merge libraries but only originals (if unedited) or edited versions but not both of the same photo. It also does not merge albums or folders, just photos.

Here's some food for thought:

If the combination of libraries would be less than 200 GB you could combine them all into one and put it on a 250GB OWC Envoy Pro mini which would let you carry it along with your laptop and have all photos in one library for easy searching.

Good Luck! PowerPhotos is a great companion for Photos. You will need the full, paid version to merge libraries and find duplicates, but even the free trial version is great for displaying the metadata with your photos. What I am missing most in Photos, coming from aperture, are the list view and the metadata displayed below the thumbnails. PowerPhotos lets us view the photos in nice table view with the metadata displayed I the columns, so we can sort the photos by the file size or the filenames, etc.

Or as thumbnails with two different kinds of metadata below the thumbnails. and it makes it easy to keep the photos in separate libraries, because we can view one library in photos and another in PowerPhotos, very flexible.