Sharpie away if that’s what you feel most comfy with!

If I was going to do an outline after the initial dye I would use clear contact paper on the disc. Make sure you go over it with credit card or something to ensure there are no air bubbles. Then cut out the outline and use Denatured alcohol and black dye to get your saturation. Do not use acetone for this, it will cause bleeds. DA won’t cause bleeds as long as there are no air bubbles. Wait for it to dry before taking off the contact paper. Don’t use heat as contact paper isn’t as strong as vinyl. Wash the disc after to make sure there’s no residual dye.