How can I read file content text / retrieve the content only of the object / selected item in the newest UI VAF

The next (25.12) release of M-Files will bring the DownloadFileAsDataUri  method to the UIX.  If the file size is not too large then you could use this.  The public documentation will be updated as we move towards that release date.

Alternatively, and perhaps the best way, is to use a vault application (VAF application) to do the reading.  Expose a vault extension method that takes the object and file data to process, process the file within the vault extension method, then return the processed data back to the UI application (again, for example, as a base64-encoded string).