Good news, sort of! As I just posted, the work to update to TexLive 2020.0 is more or less complete, I hope, and ready for testing.
As part of this work, I did a lot to tidy up the way that I’ve been generating the bundles. This is found in the new repository tectonic-texlive-bundles and does a lot to improve the bundle-creation process. It still, however, depends on checking out the TeXLive Git mirror, which is quite large. I hope that this will lower the barrier-to-entry for folks to contribute to the bundle creation process.
Anyway, I would be very interested to increase the level of automation in this process! Some care is needed because changes to the included files sometimes interact with updates to the core engine code, and updates to that are not automatable. But with a good test suite, we more frequent bundle updates should be pretty safe.