Your assumption that such a formula is "easily findable" is false.
Hash functions effectively scramble the data so there's no way to predict what the output is going to be,
so you cannot find a "progression rule".
Correct, the consensus is that good hash functions are "one way". They require a ridiculously huge amount of computing power to brute-force (predict), but only a small amount to verify information.
Okay, that is clear, but a hash is not a random number.
More significantly, there are a number of ways to go from address to private key.
Finding a precise formula would be extremely difficult of course, but would let you go from address to key in one step.
However finding only the relative position of an address, being able to say one address comes before or after another, would be much easier and would get the private key of any address within a few hundred steps by telling you whether you need to generate a higher or a lower private key.
So the hash itself contains several different kinds of information that together may give the appearance of a random number. But if you take a string of several private keys in proper sequence from low to high, then generate addrresses from them, and there is any corresponding pattern whatsoever in those addresses, you do not need to find any formula to solve the problem the second way, which takes a few hundred steps.
I hear what you are saying, but the fact of the matter is that so far, no one has
been able to detect any sort of pattern in strong cryptographic hash functions
such as SHA-256.
There is something called the 'avalanche effect' where changing one character
changes the outcome completely. If you look into the inner workings of the
hash function, it goes through I believe 64 rounds of computation. By the
time its done all that computing, you're left with something that has no
discernable pattern.
So while you're correct that they are not 'random', the outputs appear
random for all intents and purposes and without a pattern to follow,
no ordering is possible.
Perhaps someday someone may indeed find a pattern, but when/if
that happens, it would be time to migrate to a stronger form of
cryptography.