A few big changes that I would make to this list:
1. I would not recommend running a 20 man plague marine squad. it makes them a big target that is an easy choice for assault cannons or high yield medium strength weaponry. This allows you opponent to easily direct their fire towards that squad and not worry about wasting any shots. Furthermore, I would not recommend you put any melee weapons on your marines, they are too slow without transport, most people would shoot them anyways, and plague knives are already above average for weapons. If you want to run 20 plague marines then run 2 squads of 7 and one of 6 fully kitted with blight launchers for about 400 points total. This forces your opponent to spread their fire, and with
DG sustainability they will waste shots, while being offensively powerful and a pain your opponent must deal with. Plus you have extra hardy troops for back and midfield objectives
2. I would drop your Putrifier, and Blightspawn you simply do not have enough chaff (ie Cultists/Poxwalkers) to protect them long enough for them to get into range with their mediocre maneuverability (Even with advance) to use their abilities effectively. Your Blightbringer can add maneuverability but not to the range where your other units will be useful, but it is still good for moving range Plague marines around. I would instead run a plague surgeon to make your
DG even more durable and even more of a problem to deal with. And every shot toward your plague marines that is less on mortarion.
3. I would also recommend you drop one plague drone in order to get another Demon prince to upgrade the spearhead to a battalion, this should leave you with about 18 pts which you could spend on your plague marines in the form of a champion with a plasma or meltagun.
4.
IMHO epidermis is not a good unit because his buff takes forever to get going and you would get the best effects around turn 3-5 which is far too late and that is highly variable based on who you are facing thus with no consistency it is not a great unit. Plus mortarion provides all the buffs needed to
DG with is Warlord trait, and auras. (Certainly your nurglings would not need the buff)
This all in all would leave you with a strong backfield with plague marines with Blightbringer for movement, plague surgeon for increased sustainability, nurglings and 2 casters. And a very strong end field assault group consisting of mortarion, 2 drones, and 2 Demon Princes.
With this you have a fun list that can hold its own against others pretty well. Good Luck