I don't think the spread rate of any religions should be decreased.
But I think the tech paths to order/empyrean/av need to be shorter. It's much easier to found Empyrean in FF, I think.
Allowing the others to be founded earlier would help, I think.
But aside from that, there does need to be much stronger AI weighting on religions.
I think certain civ/religion combinations should be marked as Abhorrent, so that they will never switch voluntarily, and can only be made to adopt the religion by force. If they're losing a war and it's a condition for peace.
Examples
Bannor/Elohim - Ashen Veil
Svartalfar/Ljosalfar - Runes of Kilmorph
Malakim - Council of Esus
Sidar/Svartalfar - Empyrean
Sheiam/Hippus/Lanun - Order (the last two because they're undisciplined peoples who enjoy freedom, rather than because they're evil)
Khazad/Luchuirp - Fellowship of Leaves
Aside from that, each civ needs more weighting towards their preferred religions, so if a religion they really like spreads to their land (ljosalfar + FoL) they're much more likely to dump their current and switch to it.
Also, I think the Sheiam need a farther special exception, as they depend so much on Armageddon. They should always beeline corruption of Spirit ASAP, and once they get AV (whether or not they founded it), switch to it instantly, and mark all other religions abhorrent so they can never switch without coercion.