These are the sub-projects that we are currently building and testing on the release branch, so I have confidence we can start testing them on the main branch immediately.
We chose these projects for testing on the release branch, because at the time, these were the most the we could build without running into the disk space limitations or the 6 hour build timeout.
The default github runners have more disk space and CPUs now, so it’s possible we could add more, but I have not tested this.

If the tests are slow (comparatively speaking to the build), have we considered a “build-only” CI? (the “build” is cacheable, the “test” aren’t with our system: making the tests a likely bottleneck).
This is an option. It just depends on if we value the lit tests more or less than build testing other sub-projects. I, personally, don’t have a strong opinion on this.