Seeking cheaper photo software alternatives for new iMac

To add to the advice by Limnos to use the included “Photos” app on your Mac (it is more capable than it might first appear). There is also the newly released version of Affinity Photos (now called Affinity Studio). It is quite powerful, and probably has less of a learning curve compared to Gimp. There are hundreds of YouTube tutorials for the new software.

https://www.affinity.studio/

Photos is an Image Management system, not just an editor. Finder is a File Management System which lets you change file names and keeps track of modification date and stuff associated with files. But images are way more than files, and Photos organizes pictures with creation dates, captions, GPS locations, faces, and more. To do this, Photos needs to have control over the pictures, so they are Managed by being copied into the Photos Library where the database is stored. The Photos Library of pictures can be large, and it can be held on a directly connected external drive-- but not networked, clouded, NASed, etc. Photos interfaces with iCloud Photos, so that all the pictures are available on other connected devices.

Photos is a non-destructive editor. If you edit or crop a picture, maybe cutting off the sides or intensifying the color, the original file is never touched. Instead, your editing steps are stored in the Photos Database. It's the same for every kind of edit, keyword, comment that you do-- the original picture is not altered, but the information is stored in the Database. So the picture you see on the screen never existed as a file-- it is constructed on the fly from the original plus the information in the database. 

I also use Lightroom Classic (subscription, bah!) and other editors, but Photos does most of what I need.

I keep my "Library of Pictures" divided into multiple separate Photos Libraries, some of which are entirely archival. I have a Photos Library of only favorites, for instance, that connects to iCloud. I also keep backup copies of all the original pictures on separate hard drives.

Gimp

The advantage with using Photos is it interfaces with Apple's other services. To edit a photo, though, I suspect the copy would have to be local.

I almost never use it and I agree that the interface is complex. I think I used it to color correct some 45 year old faded photographs I had scanned, but that was it. On the other hand, it is free. If one wants free then one's options are limited. In that case you may wish to do a broader entire-web search for software that meets your needs.

I haven't actually opened Photos on my computer yet (on my old computer iPhotos was not available since Apple had pulled a compatible version from the store). It may have some tools. Others seeing your question may be able to provide suggestions if you mention for what features you are looking.