There's a lot to unpack there.
Originally, it was one big cult, formed during their Renaissance, from people losing faith in existing gods,
So you had a religion forming in opposition and compensation of the loss of the existing one. What is the main religion before the Renaissance ? Is its teaching different or similar to your cult ? Is it a more fanatical version of that main religion ? Or is it a replacement ? (For real life example, look at scientology, that have little to do with christianity, and mormon that are hyper christian).
but now it exists as several small cults, unique depending on their region and local culture.
Are those smaller cults just regional chapters of one big cult ? Or are they the result of multiple schisms in your big cult that result in your regional cults as likely to be friends than foes depending on the particular schism ?
They promise an eternal paradise
That probably doesn't happen just by joining the cult, but by following principles, teachings, rules or other ways of life. What are those rules ? What happen if a cultist doesn't follow up on that ?
While the founder was the stereotypical madman, the future leaders are cunning, charismatic, and have a cult of personality (no pun intended)
You may think of them as a madman, but for the first generation of cultist, they were a prophet. What did they left behind to study ? To teach ? What drove peoples to join at that time ?
Also, when did the following leaders took their place ? Do they act as a group, maybe with some kind of "board" and "director" (scientology with the Watchtower society) or are they just the leader of their local chapter / fraction / branch, or are they the one leader of the whole cult, but have to deal with a cult fractured between all those smaller cults ?
The cult keeps out outside information (and most branches do this by targeting isolated villages and communities)
Do they target already isolated communities and make sure to keep them isolated ? Or do they create those isolated communities by bringing members to build and live in new villages in remote places ? Do they keep contact with other cultist communities ? How do they integrate with the local power if there is one ?
The cult has the appeal of being against murder and executions and having support towards rehabilitation, though by "rehabilitation", they mean torture, brainwashing, etc.
How is one better than the other for a cultist ? How is the other one better for the non-cultist ? Is there debate on this issue outside of the cultist/non-cultist or is it something already divisive in the general population ? Do the cult just hide cultist that should be executed by the local law from the local law enforcement ? Do they actively fight against the law enforcement ? Corrupt them ?
All those questions will help you have a clearer idea of the cult and it's values. Some answer may even be incoherent with the others as long as the cultist have an explanation to that incoherence ("thou shall not kill" is a christian value, yet death penalty was/is still happening in christian countries, with various explanations on how to reconcile that paradox)
It will also help you better understand how your cult meshes with the outside world, and if/how it clashes with it.
The common point between all of those questions is, how does your cult meshes and interact with your wider fantasy world ? Cults are often not believable because no one join something obviously and utterly evil just for evil sakes. The cult itself may be full on evil, but it still need an internal justification for it.
In short, you need to ask yourself why they drink the kool-aid (sorry for that pun). The original one was that they weren't supposed to die, but to join aliens gods instead, and drinking was an act of faith, at the end of a long time slowly recruiting and persuading and promising heaven to those peoples, convincingly enough to make them either bring their families or cut themselves out of their families.
Maybe your cult bring medicine and order to very remote places that are not helped in other ways ? Or they bring a renewed faith to desperate peoples ? Or community to people that are cut off from other newly atheists and religious peoples ?
For the rich and powerful, maybe it bring access to important political or business allies that are themselves cultists ? Or new ways to assert their power in places where they couldn't project power before ?
You may also watch this: https://www.youtube.com/watch?v=nyL8FH3Su4M (Terrible writing advice, episode on cults)