TYPO3 Core

I just tried setting editor.config.toolbar.shouldNotGroupWhenFull = false (RTE yaml configuration) but this value is hardcoded in CKEditor5Migrator::migrateToolbar()

Instead of using the CKeditor 5 setting editor.config.toolbar.removeItems (https://ckeditor.com/docs/ckeditor5/latest/features/toolbar/toolbar.html#extended-toolbar-configuration-format) you have to use CKeditor 4 setting editor.config.removeButtons.

What about adding an option to disable the CKEditor4 to CKEditor5 migration and simply add the full yaml configuration (except those settings which cant't be defined via yaml like content and UI language) to CKEditor5?