One of the awkward things about this pull request is that it proposes to have the original OSM Carto coexist with a derivative style that superficially resembles OSM Carto. Ignoring that issue, I suspect many mappers would perceive the style as a temporary step backwards even with the inclusion of busways. Both the style and OpenMapTiles would require quite a bit of work to achieve feature parity with the original style. Is the OpenMapTiles project prepared to undertake that effort? (Another OSM Carto derivative, Apache Baremaps, has some room for improvement as well.)
The more modern vector tile stack potentially addresses some of the underlying technical debt in the original codebase, but it doesn’t by itself address the difficulty of evolving the design to accommodate an inevitable progression of new ideas from the OSM community. Even OSM Americana, young as it is, has been in need of a talented designer to set the project on a sound path. But the “OpenMapTiles style” is more complicated. It inherits many design decisions that assume the capabilities and limitations of Mapnik and CartoCSS rather than those of MapLibre. Would an OSM Carto lookalike style be open to departing from the OSM Carto look and feel to continue improving? This is the difference between using OSM Carto as a steppingstone and merely simulating OSM Carto as a demonstration piece.
Completing option (1) certainly would ease the pressure that OSM Carto is facing, which would not be a bad thing. But it raises more questions than answers in my opinion.