Now that the Society vs Spellguard plot is over with, perhaps a prelude in the starting area could be set up that automatically sets people up in the Lower Cults.
A great idea, a prelude that gives you three options to choose from that will put you into the cult's base and automatically puts you into the faction would be a benefit to all cults for characters that want to just start out in a cult. A cool idea would be to give points like the lower start and to have cult-themed loot items to pick from for each cult.
I like this and think it might be a nice touch for associations in general
This is probably better handled IG than through customized preludes. The problem here is that you're affording PC's access to resources that they maybe have not necessarily earned and can potentially lead towards abandonment of the cult after those resources have been utilized. Just like there has to be a PC recruitment process for Watchers and Auxiliary, there should be a PC-driven recruitment for these cults. Unless, of course, there are no current PC members, in which case that can be easily solved with a PM to a DM.
Resource stealing can be circumvented by giving storing chests a key for more established members. Lower already has a free resting area that everyone uses and cults provide few resources beyond those two things. Maybe it shouldn't necessarily be a prelude in the starting area but an NPC recruiter in their respective faction area. Regardless, I don't foresee the cults being properly represented without some incentives for them to be so.
If this were implemented I would wholly recommend moving cult storage to locked rooms