Including HTML content from WordPress when it’s not your post content but your theme settings can be done! It’s just a mess.
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Saving Theme Data to a File

Including HTML content from WordPress when it’s not your post content but your theme settings can be done! It’s just a mess.
If you’re confused about the weird “ES” names with regards to Gutenberg and Javascript, there’s some history that may help you out.
Adding buttons to a Gutenberg block can indeed be done. Dare I say it? Simply.
Another version of a repeatable spoiler block with Gutenberg.
Some more extra things with Gutenberg that are on the wild side of advanced.