Base, it provides +8 per city. If we assume +25% for various percentage bonus (which is generous, percentage bonuses are scarce in the game), that's +10, or +60 total.
Spending 3,000 culture to get +60 is not a good investment at this point in the game. It takes 50 turns just to break even.....and then many more turns before you actually see a net benefit.
As for boredom, boredom is rarely a big unhappiness driver through most of the game. Border Growth....if you have a city that can afford all of those culture buildings its border growth is fine....and money takes care of the key tiles you need at that point in the game.
The tenets that throw in free buildings are nice because its an immediate benefit, and saves you hammers that can elsewhere. Tenets that give big faith/science/gold whatever are nice, because you can convert culture -> other yields. But investing culture....just to get more culture, is fine early game but not a good investment late game.
Why are we giving external routes the break, and forcing people who want to use internal routes (which frankly at this point I still think are weaker in general) to have to wait until the tree is completely filled out? Just combine them and be done with it.
