Hey,
like you said it’s currently not possible to use the same approach as for v3 (overriding something in admin folder does not work for plugins).
The solution is to use ‘patch-package’.
I answered an issue for ckeditor-plugin in bit more detail. You can check it here: Heading is not properly reloaded · Issue #23 · Roslovets-Inc/strapi-plugin-ckeditor5 · GitHub
Hope it helps (even if my answer is quite late 
)