Admittedly it's been a while since I've been on a jail broken iPhone, but AFAIK custom ROM support is almost completely missing from iPhones. The jailbreaking ecosystem there is more comparable to Xposed tweaks and root access on an Android device. Custom ROMs are far more extensive with their own software and theming frameworks, perspectives on usability and performance, and often completely different OSs (that are not Android) thanks in part to GSIs these days.