I am often disgusted by the range of bad information that is posted online and shown in YouTube tutorials. Dracaenas are grown regularly in nurseries and in standard potting mixes, not that strange combo that you found. A standard potting mix for most indoor plants will have perlite mixed in to provide proper aeration. Standing water in a pot only occurs when the pot has no drain holes. Such pots should not be used regardless of the potting mix.
No, don't prune the roots! Leave the roots as undisturbed as possible. I am suggesting that you get back to where you started by undoing what you did by simply removing that potting mix. After removing that soil, put the original rootball that you left intact back into its original pot. I did not need a larger pot in the first place.
Fertilizer should only be used with healthy plants that are growing vigorously. It is not medicine for ailing plants.
I'm sorry you have been poorly advised elsewhere. It is a common problem.
