With the latest update, the new default RTE library for Apex is now the CK Editor 5, the previously available TinyMCE has been marked as deprecated and can no longer be accessed where it isn't already selected.
Now, the TinyMCE instance could easily be accessed in JS via tinymce.editor to get its rendered contents (e.g. to create a preview). CK Editor has a similar function, getData() to get the content of any given instance - if you know what the instance is.
There is a global object called CKEditor5, but trying to call the getData() function on it just leads to an error. I also know I could get the raw text value in markdown (so with ** for bold etc) with $v('P123_ITEM'), but that's not too helpful.
In general the documentation around the new RTE library is quite lacking / nonexistent - anyone have any idea on how to get the instance of the active editor?