That's why I value representation. It gives me +3 happiness in my 6 largest cities (I usually play the bigger world sizes), and has the same upkeep cost as despotism (which provides no bonuses).
Another concern I have is that by allowing other religions to come into my lands, I end up giving other nations free money.
I've never really tried it, but I could see an advantage of going for respresentation and mercantilism, while maintaining closed borders. I can't remember if you can still trade techs with closed borders though.
How much will opening the borders for trade help you at the moment? Do you have any connections to anyone outside of along that river?
EDIT: I'm pretty sure some mods have come out to modify the attributes of the religions.
I don't blame them for not touching that subject though.