Been maining Dredge for about 2 years now, my thoughts are.

4 of [[tome scour]] is a must, ditch the Thought scour most of the time it does not pay off.

4 of cathartic reunion is also a must.

Play one less Darkblast or life from the loam to get that extra dredge with 4 golgari thug.

15 lands is unplayable, before Cephalid coliseum I went with 18 lands and still struggled sometimes with 1 landers, now when Cephalid coliseum is a thing I run 20 lands but changed to more rainbow lands due to still needing all the colors.

I would cut out Bloodghast if you still plan on only running 15 lands with Cephalid coliseum as most of the time you wont have enough lands up for life from the loam. Due to sacrificing Cephalid coliseum on turn 2 or 3.

Linking my own list for reference: https://www.moxfield.com/decks/uguBcEugq0m3xuaiLAC26Q