import Aperture photos into Photos app

Can you upgrade to macOS 13 Ventura or later? Then you can select your Aperture Library in Photos and use "File > Import" to import photos directly from the Aperture library to Photos. I may take a long time, however, if the library is large. Photos will show you a list of photos that have not yet been imported. Then select the photos you want to import from this list.

On macOS 10.13 High Sierra you cannot import directly from an Aperture Library. You would have to export the photos to a folder using Aperture to export them, then import the exported photos from this folder.

Or create a Photos Library from your Aperture Library by opening the Aperture Library in Photos.Launch Photos while holding down the options key ⌥, then select your Aperture Library in the Library Selector, when it opens.

While you are still have access to Aperture on your old Mac, you may want to use it to create smaller, partial libraries from any huge Aperture Library you may have. In Aperture we can easily export selected folders as new libraries. Then open the smaller libraries in Photos to create Photos Libraries from them. It is much more work to split libraries in Photos.

Many of us have multiple Photos Libraries, and I don't think that background activity is a problem. I have a Library for my wife's family pictures, some from 1860 or so, and I have a similar one for my family. I also have a Library for holding many thousands of pictures from our time together-- many of them scanned from slides and prints. I have maybe 3 Libraries devoted to more recent pictures. All of these reside on my Mac Air's internal drive. Most of these Libraries are pretty static, even though I go back to them from time to time to ammend metadata or add new pictures I'm finding. There may be a need for a good bit of empty hard drive space to allow for memory swapping. I often keep the computer on all night for it's background work.

Of course, I keep backups. I use Time Machine to automatically back up the drive, and I also periodically copy the libraries to another external drive. And on other drives I have these pictures saved in folders or pdf files.

So, if I were you, I'd keep the pictures in folders some place, and I'd also keep Photos Libraries to make management easy.