Does anyone have a precise science for all the things that determine Spire difficulty? Obviously I'd like Spire to be as easy as possible. I've heard even the AW levels can make it harder so I know there's a lot of variables.
Short answer: no, no one knows precisely.
Short-ish answer: after collating huge amounts of data, a player a few years ago came up with the discovery that the factors which affected Spire and tournament difficulty were
number of expansions placed (premium have slightly less effect than regular),
relic boost percentage (tops out at 700%),
number of technologies researched in the tech tree (optional researches have no effect), and
number of wonder levels. Obviously, of those, the only one you can "undo" is number of wonder levels.
Long answer:
this is the math on which the above conclusions are based. Most of the factors are variables, meaning they differ depending on how your other numbers stack up, and can't be quantified in a neat package. Some players think that the formula must have been tweaked ever so slightly since MinMax Gamer did his analysis in 2020, because things are marginally cheaper for them now than his model suggests they should be. Personally, it still seems accurate for my numbers, last I checked.