You seem hung up on the need to have more beakers than the AI to get an early religion. Last night on monarch I started 6 games on random huge maps, so 10 AI opponents who were also random, using huayna Capac I got a founding religion 5 out of 6 times. I didn't change my starting location or adjust which tiles were being worked, I simply went for polytheism, because those "religious fanatics" (almost) always go for monotheism cause its cheaper. Test it yourself if you don't believe me.
On Emperor and above, the probability is even less. And when you say Monotheism is cheaper, I am assuming you mean Meditation. Monotheism requires BOTH Polytheism and Masonry, which also requires Mining.
On standard map settings on normal speed, it usually takes up about 13 turns or so to pop Polytheism if the 1st tile you are working on is not a commerce tile, which is very often. That's a LOT of turns at the beginning of the game that you are spending that could have gone to more useful techs to grow your cities and expand.
As for missing out on improved tiles. I assume you build a worker straight away if your desperate to improve the land asap, it only arrives 3 turns before polytheism is done, with Huayna Capac you already get agriculture, and odds are one of your strategic resources will be improved with agriculture (every map I started had at least one) so send your worker off to improve that. You still only have a population of 1 because all your food went into getting your worker, so while you wait for a second pop, and so the need for more than one tile improved, you can develop which ever worker tech you need (if you need any beyond agriculture for your first few pops). Even if you aim for animal husbandry which is a 2nd tier tech, there is only 4 turns between your second pop and your worker improving your cows or which ever. So the cost of the religion is at worst the bonus of improvement on one tile for 4 turns which could be as little as 4 food bonus at most about 12 food, compare that to the potential of literally 100's of gold every turn from the middle ages right through. Yes I would call that a minimal investment. And if you build anything other than a worker first then both techs will be completed before you even get a worker.
Once again on standard map settings on normal speed on emperor, you usually get Poly 2 or 3 turns before your first worker pops up, assuming your 1st tile being worked has no commerce.
With Huayna, the only worker tech you have at this point is Agriculture. Each farm takes 5 turns. Assuming you are given 2 farming resources and counting time to move your worker around, roughly 12 or so turns in and you will be done with improvements with nothing else to do for your worker. By then, you had 12 turns + extra 2 turns that you got Poly before your worker was built, into researching some other worker tech. You will be lucky to have 2 worker techs done at this point... usually just 1 done with another on the way. If you have a start with lots of forests around, you are doubly screwed since you will need to wait many more turns to get Bronze Working done just to chop the forests to build something else afterwards.
Notice that you do not start with Hunting with Huayna in your example, so your Animal Husbandry will be more expensive to tech if you don't go for Hunting. If you have no resources like fur, deer, or elephants around your capital, it is a waste of turns going for that since you already devoted 12-13 turns researching Poly. And for Animal Husbandry, that will be another 13 or so turns you will have to wait.
And that is probably one pig or sheep or cow tile around your capital at most. Once you are done with that, then what? What do you do with your other tiles? Do you let your worker sit idle? Or do you delay even building your 1st worker until 20, 30+ turns have passed?
By the way, you won't be able to cottage immediately either. Sorry you will need wheel first which will also eat up 8-10 turns or so, on top of Pottery which can take you roughly about the same turns. And while you can certainly say that you can spend your worker turns building needless roads, I would say that is really suboptimal.
To me, this kind of strategy is just too situational in very special cases where you are lucky with your starting location in resources and tiles around your capital. Of course, every game of Civ 4 gives you interesting choices because every game is different, but this strategy isn't to be regarded as a standard strategy. You will need to make sure that you are not wasting your worker by letting him stand idle with nothing to do. And delaying workers for too long and working unimproved tiles for too long is a dumb way to go in my opinion.
When you use the words like "desperate to improve your land", I see a problem with that. I consider it as a basic necessity that needs to be done every time. Working unimproved tiles is a no no. I don't know how you play your games, but I reckon that it is vastly different from how I play mine.
Notice the word "potential." Founding your own religion never guarantees that you will get hundreds of gold in the middle ages. You could end up being the lonely heathen while everyone else is happy running their own religion different than yours. And if they start using Theocracy to block your religions from spreading, good luck.
Do you spam missionaries and try to spread your religion to the AIs? If you do, I think I would rather spend those hammers into something else. There is just too many things to build.
I don't see why a great priest has to be your first GP. You could get it at 1000 AD if you want, and it will still provide a huge bonus every turn thereafter. In fact I would prefer it not to be the first GP because at that stage most likely only a few cities share your religion. But a large amount of the first wonders provide you with priest/prophet GPP so its likely you will get one sooner rather than later. The wonder that creates your first Great Scientist points isn't until the great library, so unless your running specialists, a Great scientist isn't going to be your first GP. Running specialists early is a whole different debate, but not my preference.
If you are so bent on founding your own religion, I am sure you are talking about shrine income. To get the shrine, you need a Great Priest. Since every Great Person becomes more expensive, I would think that earlier the better to get the Great Priest while it is cheap. I certainly do not want to have spent so many Great People points to pop a Great Priest, when I could have gotten a Great Scientist to build an academy or bulb a tech, or even settle him if necessary. Even Great Merchants make more sense to me in the later game than Great Priest.
And having an earlier Great Priest means earlier shrine, which helps spread your religion sooner. Too few cities with your religion? Then you definitely want the shrine earlier to help the religion spread earlier, not the other way around. If you wait too long and start having rivals that have had other religions spread in their territory, it will be more difficult to spread your religion. And like I mentioned, good luck if you waited too long and the rivals are running Theocracy.
Relying on wonders to get your Great People points is something I did when I was playing Chieftain and Warlords level. It is simply too inefficient. You build wonders for its effects, not the Great People points, which I consider to be just an add-on bonus on the side. If you wait until the Great Library to start accumulating points toward Great Scientist, I think you are really wasting the potential there.
Anyway, Like I've said previously, this isn't an "every situation" strategy. One of the great things about Civ IV (and the reason we are all here discussing it 6 months after Civ 5 was released) is that every scenario potentially requires a different approach. And I was simply trying to point out that because of this, grabbing an early religion shouldn't be automatically discarded as potential starting play.
No it is not. In fact, I would say that it requires precisely certain conditions that occur very rarely unless you alter the map. This "strategy" to me has potential if it works out, but is too risky and doesn't guarantee the kind of results you will be hoping for. It also sacrifices too many little things that help the players play the game better, which are often overlooked or not realized by many players including myself, who are still trying to learn these little things that matter a lot.
Just take a look at all the posts and write ups and videos made by high level players playing Deity and Immortal and such. I have not seen ONE of them praising going for early religion. There is a reason for it. At lower levels, it certainly doesn't matter as much as I have done this myself at Warlords and below levels.
I'd love to see one of your game saves (including the 4000BC initial auto save) sometime, hopefully the ones that have the World Builder locked so that there is no doubt that there was any alterations to the map and the game.