Read Getting Started with Block Themes by Ben Dwyer (ThemeShaper)

The rationale With Full Site Editing on the horizon for WordPress, Theme creators need to start to learn how to make themes in a different way. Full Site Editing is sea change in the way that themes work. When Themes were first added to WordPress, they were simple; just a few template files and some...

This makes it seem like new themes with Gutenberg may be easier than I would have suspected? I may have to try this out soon, though I should also look at some of the newer themes that are out that use similar set ups.