I solved it by removing the code from the docs and installed the plugin https://github.com/Roslovets-Inc/strapi-plugin-ckeditor5.
Then I customized that using the customizations (toolbar customizations and custom styles) I had added to the code before.
I also had to apply this fix https://github.com/Roslovets-Inc/strapi-plugin-ckeditor5/issues/12#issuecomment-915618516 to make the media library button work.
Now it’s working fine.