This deck really desperately needs more of the defenders. Murmuring Phantasm is really good at that, so make it more than a 1-of. You truly do not want even a single copy of Paranoid Delusions. Cipher wants evasive creatures to attack, which is wholly antithetical to a mill strategy. You really don't have enough of a constellation theme to justify the Thassa's Devourer and Riptide Chimera combo, so you might want to try the deck without them. Radiant Fountain is also simply bad. It barely gains you any life, and only producing colorless mana makes it terrible. Diabolic Tutor has a double B in the cost, which makes the mana colors much harder. You really need more Swamps or black sources generally. I'd recommend looking into the transmute mechanic from original Ravnica. They're in Dimir colors already, so there's no need to readjust your mana base to accommodate them, and they're generally on the cheap side as far as tutors go. Also: removal. Get 4 Doom Blades and 4 Unsummons. Some creature removal will draw out the game way more than any number of Cancels. You have a good instinct with the defenders, though; just play more of them. Hopefully, nothing I've recommended is too out of your price range. Happy deckbuilding.

You do NOT want Last Thoughts, just a handy tip there. It's too expensive for a single card, and exiling it just to put it on something that could be destroyed (Unless you put it on something like Darksteel Myr) is just a bad omen.

Get Wight of Precinct Six and Essence Harvest

Wight is a poor man's aberration and essence harvest is a second win condition with either of the before mentioned cards

I love this deck!maybe a Jace, Memory Adept would help?

I used to keep him on the sideboard, but he just costs too much (5) to be viable. Ideally this deck should win by turn 4-5, and there are much better spells that are cheaper and do pretty much the same amount of mill as he does (10 mill can be achieved by Glimpse, 13 by Archive, 8 by Breaking, etc.).

Breaking is a 4-of. Almost as good as Glimpse the Unthinkable and a lot cheaper. I'd run more fetches. You want to trigger Hedron Crab as much as possible and more fetchlands do just that. Maybe 2-4 more fetches? This deck I think could use 2 more lands as well, but I'm probably wrong on that.

You need 2 more Mind Funeral urgently and 2 more Glimpse the Unthinkable but it's expensive. 2 more Visions of Beyond is a great idea too. Good luck!