I’ve just released a new version of subiquity to the stable channel, 20.03.1. You can see more details at:
(19.12.2 never got released to stable, although it was on the focal dailies).
There are an assortment of bug fixes, an improved UI for the guided storage options and a substantial rewrite of the internals that should have no user visible impact. There is some support in this release for autoinstall but that’s not really yet ready for wider testing. Soon!