might be a silly question, but isn't macos relatively easily virtualisable? i understand it's not done commercially bc of licensing issues, but is there a piece of software where you just get a vps or two and virtualise all of your build targets yourself, so your ci bill is just the vps bill?

Virtualizing macOS on non-Apple hardware is illegal. That, and in between the costs of hardware (even assuming I breach their licence and go non-Apple) and the costs of electricity, this quickly becomes a sizeable time and cash investment.

VPS bills accumulate. I must have saved hundreds or thousands of dollars by not hosting this myself over the years.