All of those things are in Anomaly. Some guy joined my colony with Deadlife Dust, an AoE ability to raise all humanoid corpses into zombies. Another game I had a dude who could turn a corpse into a hideous flesh beast. I've studied the monolith and unlocked dark ritual research tree with all sorts of stuff like you're asking about. Arcane books are also in Anomaly, as is the ability to write your own books.
It's everything you're asking about, including some new building stuff to make spooky cabins etc. It's what the DLC is built around. Everything unfortunate can be used to your advantage, such as components for dark rituals and monsters to unleash on your foes.
You can just disable the annoying events, same as any of the annoying events in base game like crop blights.

Check out psychic rituals on the rimworld wiki.

all of those things are part of the ROM Cults mod.

just go to your steam library, rightclick RW, search in it's settings for "BETA-versions" to be allowed, then select 1.43 from the list and you're good to go to play with it, just without all the anomaly/odyssee stuff ;)

i've had some great fun in the past with a village of 25 peasants working their a55e5 off to please their vampire lords in their castle's tomb, guarded by 3 revia bodyguards, 1 of them cyborged, another one had all her body parts replaced by tentacles given to her as presents from cthulu. cults mod is really amazing and i miss it horribly. it has functions to set up religious messes in the temple, it demands HUGE temples, you can sacrfice ppl on altars to get all kinds of stuff. shub'niggurat spawning a horde of black goats to feed your colony, zathoggua gives potions and sleepertotems (great thing to stop prisoner escapes), dagon lets a titanic-like shipwreck crashland nearby from out of the sky to harvest the precious stuff inside (gold jade plasteel whatever valuables u got active)....i don't wanna spoil it all now, there's amazing stuff in it and it's a true nightmare that jecrell/ryoma quit modding.
despite it being for RW v1.4, you should give it a try and enjoy. if you don't, you've missed out exactly what you're looking for.
waiting for the mod to become revived is also not really an option, since it might take several years (without AI slavework)

How much of CoC Cult mod do you think should be resurrected for a more vanilla-integrated mod? As in, bare minimum. Do you think a bunch of it can simply be rolled into existing systems with new anomalous entities, dark rituals, implants, genes, etc?
Those elder gods sound pretty cool, how does that work? Some of what you describe has a bunch of mods and DLC covering it, but it sounds like there's a fair bit of cool stuff that does more than simply create an eldritch cult theme.

Sounds feasible to crib together a small mod with the most interesting and coolest-looking critters, some new rituals, ideology style, etc. and a few recommend mods to fill in blanks.

Here are some options that might fit your “evil cult” theme:

Call of Cthulhu – Cults (Unofficial Continued) – Lets you create cults, perform dark rituals, and summon creatures. Works with newer RimWorld versions.

Cosmic Horrors – Adds Cthulhu-inspired monsters that can be summoned or encountered. Pairs well with Cults.

Custom Ritual Framework + Sample Rituals – Lets you create your own rituals, sacrifices, and arcane effects. Great for DIY occult gameplay.

Summon Colonist Ritual – A recent mod for summoning colonists with ritualistic effects, adding a creepy, mystical vibe.

Controlled Rituals – Ideology – Lets you control ritual cooldowns and durations for a more balanced dark-magic experience.

If you combine Cults + Cosmic Horrors + Custom Rituals, you can really create that cold, dark, evil cult atmosphere you’re looking for, with summoning, zombies, and arcane libraries.