As mentioned in the comments, it isn’t currently supported, although there is an open issue for exactly this feature: https://github.com/WordPress/gutenberg/issues/17099
Meanwhile, I can think of a couple of ways to achieve the same effect:
1) Pages + custom rewrites
As you suggested in your question, you could create a set of parallel pages with the same slugs as your custom taxonomy terms. You can then either:
- Change your theme’s custom taxonomy template to pull the data from pages with the same slug, or
- Add rewrite rules to redirect term urls to pages with the same slug, using built-in wordpress functions rather than the .htaccess (see https://developer.wordpress.org/apis/handbook/rewrite/)
2) Standalone block editor + custom logic
It is possible to create standalone instances of the block editor. Once you’ve achieved that, you would have to develop custom logic to:
- link the data saved by a standalone instance to a custom taxonomy term, and
- pull that data from your theme’s custom taxonomy template
Here is the explainer on creating standalone block editors:
https://developer.wordpress.org/block-editor/how-to-guides/platform/custom-block-editor/tutorial/