It's a pretty nice start, with some key discard cards already in there. In the tests I've done, I feel the discard theme is not doing quite enough right now. You need the powerhouse cards such as Zombie Infestation to really start doing things, but you should have some more powerhouse cards. I must admit, though, that I'm not quite sure what other powerhouse discard cards would be.

I do have some other suggestions. Replace Frilled Mystic with Diplomatic Escort or Stronghold Machinist . While they don't do the exact same thing, with the Escort and Machinist being tap abilities, they do have the additional effect of adding to the discard possibilities. Frilled Mystic is pricey in terms of mana, so you'd be better of playing a normal counterspell if that is what you really wanted.

Underrealm Lich could be amazing. For every card you draw, you essentially get to draw three and choose one that best fits your current needs. It does fill up your graveyard very fast as well, but I think that'll be helpful the majority of the time.

Taigam, Sidisi's Hand could be very nice. It's essentially a card without downside, as you'll have your commander out most of the time anyway, which means it's just an extra card. You'd even be able to choose which effect to activate first, which means you draw to seven with Damia and get another card with Taigam. The activated effect is a decent way to get rid of threats on the board, as long as you fill your graveyard quickly enough. Combined with some more graveyard filling effects, it could be very useful.

Tasigur, the Golden Fang is a nice political card, and also has synergy with the graveyard theme in these colours. It fills it up, and you can make some nice deals. Should be fun.

Brawn is an awesome card in this deck, as you'll be able to use the big creatures you have, such as the growing Chasm Skulker or Oneirophage, and actually smack people really hard. It's another great card in a graveyard synergy deck, so it should work well.

In case you need more removal, Dark Withering is a madness removal spell. Not the best, but madness cards are quite rare. A general tip would be to pay close attention to the new Commander 2019 set, because one of the decks will be Madness themed, which means there'll be new madness cards and reprints soon. That's nice for the price of cards as well.

Abandoned Sarcophagus is a nice card for all the cycle cards you have in the deck, as you'll get to cycle them as well as cast them from your graveyard.

Psychic Possession is also a great card in combination with your commander, as the downside of skipping your draw step doesn't do anything for you, so instead you'll get to draw a lot of extra cards for each card your opponent draws.

Tamiyo, Collector of Tales or cards with a similar effect could be nice against a certain types of decks. cough Shattergang Brothers cough

Overall, I think that adding a graveyard theme to the deck could make the discard theme a lot better. You already have a few cards in your deck that allow you to cast or return cards from your graveyard, so why not add more and allow the two themes to complement eachother? Sidisi, Brood Tyrant could be a starting point to look for some more cards.

Best of luck, and happy deckbuilding :)