Not sure I can help with the mana base, I'm fairly terrible with that myself.
But in terms of what to cut, I'd suggest going for a bit of streamlining.

So far, the deck seems to have a very focused tactic in mind, and then some singular cards that seem to be more situational.

Civilized Scholar  ^Flip for example is a nice card. The effect help getting cards faster and can turn him into a neat little beatstick. But with only one of him in the deck, so many other beatsticks that do a better job and so many other draw / discard stuff? You might as well remove him.
Skaab Ruinator and Rancor also seem problematic to me. The former cause it removes things from your graveyard, which makes Splinterfright less useful (you should also probably be running 4 of him since he is amazing in this deck). Rancor on the other hand is a superb card, but it won't ever fill your graveyard unless discarded for no reason. And since your most powerful creature already has trample I don't really see the point all that much.
In the same manner, Ghoultree seems like another one of those "nice to drop, but mostly less effective than the cheaper alternative" cards. Splinterfright will get stronger as this one gets cheaper. He also mills you and has trample. So why not use him instead?
Spider Spawning seems like another good thing to maybe cut. It's only purpose seems to be damage prevention. Something Gloom Surgeon already does in a more synergetic manner. Maybe some more Tracker's Instincts instead?
Izzet Charm also seems a bit lonely and out of place. A second Breaking / Entering seems more productive here. In general I personally try not to have singular cards at all. If a card is only in there for one specific rare combo, you might as well not bother. Though that might just be personal prejudice. (I have literally never once drawn a Wurmcoil Engine in all my time playing Magic even though I have one in a lot of my decks)

No more time to write anything else. But I definitely like the idea of the deck and time permitting will look over it again if you want any more suggestions.

Yeah, good ol' Civilized Scholar  ^Flip ... really fits and really doesn't. The singleton is/was in here because I feel it'd be a card I'd be happy to see every once in a while, but if I get it into the grave, that's fine as well. Still not sure about it, but will probably get cut. Maybe I'll get to playtest IRL to see and find out.

I do see the problem with Skaab Ruinator . I just lied to myself to believe he would be still worth it. sigh Very powerful, but not synergetic. Cut.

Don't see the problem with Rancor though, as it is no creature card. Splinterfright and Boneyard Wurm only care about creatures. So that's a card I'll almost definitely keep! I just need the Trample as a way to get through, and the fact it's coming back to my hand doesn't prevent me from milling myself, but only adds value instead. And if I mill it out, well, sad day.

Wreath of Geists on the other hand was more of a flavor choice, that one I'll probably throw out or swap with another Rancor to make sure I'll hit at least one of those in most games.

The good thing about Ghoultree is that it's Power is permanently high and he's not affected by Graveyard hate. (Which still would annihilate me every single time.) Again, I see your point and will think about it a little more.

One Spider Spawning I'll cut. The other one I'll keep - getting 10 or more Spider tokens in one casting is just too good of a possibility which doesn't even vanish while milling. Even with the Spiders alone, I could end games, although it is more of a fringe scenario. But from playing that card before I know it's good.

Izzet Charm is really good... but I see your point, most of the time, I'd just be milling it out of my library. So it just isn't worth it. Cut.

Not sure about a second Breaking / Entering , however. I guess that would make sense, depends on how many cards I'm running besides it.

Thank you very much for your feedback!