Justin! Per usual, thank you for shining a light on this topic. I've found myself shouting from the rooftops about this so perhaps a new YouTube video needs to happen (added to the backlog).

I have not seen much appetite from developers for creating universal themes. I am not sure if any exist, at least not in the official directory.

I wanted to connect some dots here and point to https://blockbasetheme.com/ You can see some themes there that are universal themes and available on WordPress.com. Blockbase is available in the theme directory: https://wordpress.org/themes/blockbase/. Folks can use it to create their own universal child theme if they'd like: https://themeshaper.com/2021/11/17/create-a-blockbase-child-theme/