Hey @Reverentiae !
I believe it is just one or the other currently. Not ideal, but the submission comments can always be used for comments, although you may have to include a bit more text to specify what the comment relates to.
Another possible option while using the rating rubric is to use the screen recorder ability in SpeedGrader to record you comments while you are also recording their paper on the screen. Yes, this may take a little more time, which is what you were trying to avoid, but it could be something performed on an as needed basis and would not have to be done for every student. Just a thought.