Unfortunately, being on a Mac limits you a little in terms of some of the free programs that are out there; however, you may find this page useful in finding software that will work on your Mac:
https://www.macobservatory.com/mac-astronomy-software
Sadly, Deep Sky Stacker doesn't work natively in the MAC OS, you can use bootcamp or Parallels & for some older MAC OS, there are versions available in a Wineskin wrapper. Nothing available for current MAC OS... trust me, I've looked...
If you are looking for an alternative to Photoshop that is just as powerful but, doesn't have a monthly subscription fee, have a look at Affinity Photo. One off price (around $79 from memory), extremely powerful & serious challenger to Photoshop. Some Photoshop plugins will work with Affinity as well but, not all & unfortunately, there are no Astrophotography plugins for Affiinty. However, it's worth checking out if you are looking for excellent Photo developing/manipulation software.
FWIW, I use the following on my Mac:
1. ASTAP (MAC alternative to DSS)
2. Lynkeos (MAC alternative to Autostakkert & Registax)
3. Startools (Works natively in both MAC & WIN)
4. Affinity Photo
I also have a 'trial' version of Nebulosity but, haven't tried it out yet...
Hope this helps