I use On1 Photo RAW for my editing program. It’s very similar to Lightroom and has a lot of similar features (just better 🙂 to take my shameless stab at Adobe). No subscription required, but they re-release the software every year with new features. It appears the earliest OS the current version supports is MacOS Mojave (10.14). You might be able to reach out to support to see if they would be willing to sell you an earlier version? It’s a bit of a long shot, but it could work…
Lightroom, On1, Luminar, and Apple Photos are excellent photo “developers” as I like to call them. They’re great for batch processing a lot of images.
GIMP is an excellent photo “editor” to learn and get started on! “Editors” are more like Photoshop or Affinity Photo and they typically allow you to really delve and change a lot of things about one image. I tried using GIMP; it was okay. I found it not intuitive to use, but it’s a very powerful editor if you take the time to learn it.
I currently use Affinity Photo. It’s not Lightroom, but it’s very similar to Photoshop (except you only have to pay for it once and it’s yours forever).
Looks like you got a pretty good set up going with GIMP; take the time to learn it and you can get some great results!
Let me know if you have any further questions!!