Darktable is a very competent replacement for the image development part of Lightroom... It's missing a few things, but even more capable in other areas. Having previously used Lightroom 6 as my main raw development tool, I like darktable a lot (I run it on Linux Mint 18.3 and Windows 10).
RawTherapee is an excellent alternative. I'm not so keen on the user interface, but it's fine once you get used to it.
For library management, try
digiKam. It's powerful, and whilst not as slick as Lightroom (especially the UI), it does the job equally well.
If you want Photoshop-style functionality, use
GIMP. On your hardware, I'd probably suggest using 2.8 rather than 2.10 for performance reasons (I believe the last stable version was 2.8.22).
All of these tools are available for Mac OS, they're free and open-source...