The guys from Circle-CI have graciously extended our macOS build time from 500 to 1000 minutes.
This gives us about 16 hours for macOS builds.
For the documentation and static checkers, we have used 21h 33m 31s between Mar 19 - Apr 19 ...and the combine builds runs for about 5 minutes.
A single macOS build also runs between 5 and 6 minutes.
In the previous configuration, we were running the macOS tests only when the documentation / static checkers and pyflake3 linter were green.
With that into consideration, we might be able to fit within the 1000 minutes limit.
If 1000 minutes is not enough, I was thinking to configure Circle-CI to run macOS builds on Circle-CI only for PR from external branches.
For branches inside the Twisted main repo we have the macOS Buildbot builder, which is triggered automatically.
In this way, external contributors should get some feedback for macOS.