So it really ends up being too many ressources to spend. Thanks very useful to know. Thanks!
I am looking forward to trying something else then. What about Cannibalism? Doesn't look very useful apart from relieving stress and getting sick. It would have been nice to have a chance of gaining skills from it.
Also are there any special combinations which require multiple tenets to yield hidden results?
Cannibalism would be nice if you couldn't get sick and there would be a way of making all your characters cannibal. Currently I think it's not very useful for a tenet.
I haven't seen any special combinations like in CK2 so far, just some tenets working nicely with others, but many tenets have hidden features.
Picking good tenets is very situational, depends on what you want to do, what other bonuses you have etc...
In my current campaign:
I've started as Asatru - it has awesome tenets for start, thanks to Human Sacrifice I was able to get 36k piety to restore Hellenism
When I Reformed Hellenism I've picked:
Divine Marriage - it increases fertility between family members, opinion bonus is also nice
Religious Law - Allows me to revoke titles from vassal from time to time, useful in fixing my realm
Warmongering - Restoring Roman Empire requires a lot of offensive wars so it was absolutely essential.
Now when I restored imperial borders I've created new faith:
Divine Marriage - as above
Pursuit of Power instead of Warmongering - I no longer wage that many wars so I'm ok with offensive war penalty, in return I get -50% tyranny reduction which is awesome since I also own Aachen and it's special building for -20% more and my character is Callous so I got -90% tyranny.
Communal Identity - Now when my religion has spread far enough and I'm done with major expansions I think it should allow me to culture-convert faster as others have suggested in this thread, will see if it was good idea.