Comment by BrandonSmith

Continue to upgrade to the latest macOS on your host system, use something that leverages Apple's Virtualized.framework to run any version of macOS you want.

For instance, consider https://tart.run/

All the Android / iOS devs on my team use Tart locally when we need to test mixed environments.

Then we use Tart's sister Cirrus CLI to run our builds on our server.