I just want a good CI/CD system for macOS to build iOS apps without needing to buy a farm of Mac Minis, or even buy a Mac, is that too much to ask?

Then you should look at either Bitrise [0] who have a CI/CD system tailored to solving that exact problem or GitHub Actions [1] that provides macOS build machines.

[0] https://www.bitrise.io/

[1] https://github.com/features/actions