A couple of things,
Split is actually bugged right now. Enemies affected by split only take 10% less damage. Yet they deal 53% less damage. The debuff also last 10 rounds with nihil (the clones only last 7 though). It is the best debuff ever. Using it is technically abusing a bug though I guess. Oh yeah, and the clone is pretty sweet. The clone actually only does 27% less damage, not that mob damage is significant.
Also, if an enemy has 6 stacks of atrophy and you cast terminus, terminus gives you 2 stacks, if you get another free cast you are at 4, then you cast another spell and go to 6. Alternatively you just cast any other spell and go to 8 stacks. Worst case you are at 4 vs 8. So 2 turns behind. You still stay at 6 stacks of feast though because it has a duration of 5. Using up the atrophy stacks doesn't mater (much) because feast stays at the higher value for 5 turns.
Against a single target if it takes you 71% of a turn to cast a spell and your opponent has 10 stacks, terminus is essentially a free cast, and you only need 4 more casts to get back to 10. Meanwhile you keep 10 stacks of feast. So with 10 stacks it is also good vs 1v1. (it does cost 10 insanity though)
Also the damage is good!
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Does it compare to a 20 turn entropy bomb? No. But it's AoE 10! and insta cast (or better). And it only takes a few turns to build up. Plus the entire time you are gaining entropy. I love seeing the you gain 2.9 turns message.
Also, about chaos orbs. I was running into insanity problems in longer fights because I was casting things faster than they went off CD. You have a small number of insanity generators and a lot of ways to spend it. Gaining extra turn energy and cast speed was problematic even with severed threads being up. But yes, once you got going it was fine. That's because for 90% of the game you are spamming rift cutter, dark whispers, and nether blast. So for QoL I think it makes sense to dump a couple of points there.
Now if atrophy made enemies encumbered like it does players then I'd never use terminus

Anyway...
I've given your points some thought. I find your build interesting using fungal blood as an prodigy to maximize en tropic gift. However you weren't able to finish the fight in less rounds than a player who simple cast cultist spells as quickly as possible.
What i get out of that is that building around entropic gift is inadequate. Of course there is variance in the fight but i think you should play cultist again and try different ways to play it, i'm not convinced by the points you make.