Thanks for voicing your request here as well, @Mels. Reading it now I wonder if you’ve come across our library notes (pencil icon under each reference, pictured below). These don’t have any formatting tools and are connected to the reference rather than the PDF, but could still be useful for you.
Additionally, you can use the style “American Psychological Association 6th edition (annotated bibliography with personal notes)” to have these notes show up in a bibliography.
Regardless, as I mentioned via chat similar versions of this request have come up before so your +1 is already on the topic on our tracker. Please feel free to share any further questions or comments!