I would consider monumentality to be an edge case. Everything is better with monumentality, but as soon as you step outside it, suddenly the value of faith drops massively. Yes, we all know that the best strategies are built around it, but I don't know if the design of the game is such that players are supposed to plan particular golden/dark ages ahead of time.
And yes, the religion system, especially beliefs, usually end up making religion perpendicular to everything else. I don't care if you have missionary zeal and Mosques, because I'm not playing religious. If all religions had a belief as impactful as choral music, suddenly, holy cow do I care what foreign faith I have. It should not live in its own bubble so much - even if that means religion morphs into essentially a 'booster' like in civ5. I'm not even sure holy sites should be the singular expression of religion. Why can't the religious building belief go into the city center? Then pick some other thing for t3 that morphs holy sites into a district that people trying to win religiously use.
That's really the biggest flaw with religion/faith, in that it's a little of a high risk/high reward type of game. I mean, if you get the monumentality golden age then a +3 holy site can absolutely rocket you forward, since the double-adjacency card comes early, and then that saves your cash for everything else.
But if you don't have that golden age, then there's not really anything you can do to spend your faith on until mid-game. And really, for most of the game unless if you're going for a religious victory, spending your faith on religious units is actually a pretty horrible use.
I mean, I spend 300-400 faith on one apostle, who can convert maybe 3-4 cities if I get lucky with the right promotions, and what does that give me? Yeah, as mentioned Choral Music can be a useful bonus, but even something like Jesuit education only gives me the ability to spend my faith on something else.
In my last game, one of the golden ages I decided to experiment with picking the religious golden age instead of monumentality, and it really was useless. Basically, I probably bought like 45 apostles in the golden age, but to be honest, I probably could have done a better religious game going monumentality, since I would have saved on builders and settlers, which might have let me buy even more apostles. And all of that effort to spread my religion and it barely made a dent in anything I was doing.
It wouldn't be a problem, except that holy sites take crucial time and effort away that can be put to other uses. If you commit to it, a faith economy can be very strong. But it would certainly be nice if there was a more reliable way to have faith and holy sites truly impact parts of your game. For example, if you had beliefs like Choral Music but for production and science, then suddenly you can potentially use holy sites instead of other districts in the game. Or perhaps you have a belief where shrines give trade routes like markets do, and suddenly you can use holy sites as your pseudo-commerce hubs. Give me a few options like that, or maybe move the faith-buying of units as the warlord's throne bonus instead of the production boost, and suddenly you see faith economies in a whole new light.