In reply to Jack Fledgling.

Hi I'm not the author, but want to share my intake with your issue.

Most of the time you only need custom blocks for Pages or whatever your custom post type is. So limit the block to only be selectable in Pages.

Broken pages when changing theme is a normal and expected thing even before Gutenberg, so you don't have to worry about that.

But blog posts should only use built-in blocks so they stay intact.