Mirveil

I think I can more or less answer my question now...  And maybe it will be helpful to some other people, especially if they want to play with large ocean worlds.
Soil composition is dependent on a file, located at (on windows) AppData\Roaming\Vintagestory\assets\survival\worldgen/rockstrata.json
Example, basalt
        {
            blockcode:   "rock-basalt",
            "amplitudes":  [4, 2, 1],
            "thresholds":  [2.2, 1.1, 0.5],
            "frequencies": [0.05, 0.1, 0.2],
            genDir: "TopDown",
            rockGroup: "Volcanic"
        },
- Amplitude is how hard it can stretch up or down from its base.  Higher number means larger layers, like 5 cubes deep instead of 1.
- Threshold, I don't fully understand, but lower means it's more likely to be chosen to be the rock type.
- Frequency, from my understanding is like a zoom on the noise texture for the rock type... Smaller number means it's shrunk and repeats more often, thus has more chances to show up, but it makes smaller pockets.  Higher number means it's more spread out, so less likely to be present, but when it is, it takes a larger space.
TopDown means it tries to appear from the top soil and pushes downwards (with amplitude) and BottomUp is the opposite.
One last trait is how many variables each rock type has. Basalt has 3 numbers in each category, and Claystone has 4. Adding more, should mean that you can pile up more of those noise patterns, with different frequencies, to give the rock type even more of a chance to be present somewhere.
Some additional notes
- Granite is weird, I think that it might fully ignore this file and just do its own thing...
- The order seems to decide whether a rock type is likely to ever appear above another. For example you will probably never find sandstone under chalk.
- This doesn't affect the shape of the world in any way, it only affects the soil composition.
At the moment I'm using this file.  It seems to me that it's still entirely possible to find large parts of the world where it's still granite, but I think I can be more confident that I can at least find a small bit of chalk or limestone somewhere within 2000-5000 cubes. It makes bauxite, chalk, chert, limestone more common, and most of the other types less common.
{ code: "rockstrata", variants: [ { blockcode: "rock-kimberlite", amplitudes: [100], thresholds: [80], frequencies: [8.00], genDir: "BottomUp", rockGroup: "Volcanic" }, { blockcode: "rock-basalt", "amplitudes": [4, 2, 1], "thresholds": [2.2, 1.1, 0.5], "frequencies": [0.05, 0.1, 0.2], genDir: "TopDown", rockGroup: "Volcanic" }, { blockcode: "rock-claystone", "amplitudes": [5, 2.5, 1.25, 0.7], "thresholds": [3.5, 2.25, 1.75, 1.1], "frequencies": [0.05, 0.1, 0.2, 0.4], genDir: "TopDown", rockGroup: "Sedimentary" }, { blockcode: "rock-sandstone", "amplitudes": [5, 2.5, 1.25, 0.7], "thresholds": [3.5, 2.25, 1.75, 1.1], "frequencies": [0.05, 0.1, 0.2, 0.4], genDir: "TopDown", rockGroup: "Sedimentary" }, { blockcode: "rock-shale", "amplitudes": [5, 2.5, 1.25, 0.7], "thresholds": [3.5, 2.25, 1.75, 1.1], "frequencies": [0.05, 0.1, 0.2, 0.4], genDir: "TopDown", rockGroup: "Sedimentary" }, { blockcode: "rock-conglomerate", "amplitudes": [5, 2.5, 1.25, 0.7], "thresholds": [3.5, 2.25, 1.75, 1.1], "frequencies": [0.05, 0.1, 0.2, 0.4], genDir: "TopDown", rockGroup: "Sedimentary" }, { blockcode: "rock-limestone", "amplitudes": [8, 7, 6, 5, 4, 3, 2], "thresholds": [0.55, 0.5, 0.47, 0.42, 0.35, 0.28, 0.22], "frequencies": [0.05, 0.08, 0.11, 0.14, 0.17, 0.2, 0.23], genDir: "TopDown", rockGroup: "Sedimentary" }, { blockcode: "rock-chert", "amplitudes": [7, 6, 5, 4, 3, 2], "thresholds": [0.55, 0.5, 0.47, 0.42, 0.35, 0.28], "frequencies": [0.05, 0.08, 0.11, 0.14, 0.17, 0.2], genDir: "TopDown", rockGroup: "Sedimentary" }, { blockcode: "rock-chalk", "amplitudes": [8, 7, 6, 5, 4, 3, 2], "thresholds": [0.55, 0.5, 0.47, 0.42, 0.35, 0.28, 0.22], "frequencies": [0.05, 0.08, 0.11, 0.14, 0.17, 0.2, 0.23], genDir: "TopDown", rockGroup: "Sedimentary" }, { blockcode: "rock-bauxite", "amplitudes": [7, 6, 5, 4, 3, 2], "thresholds": [0.55, 0.5, 0.47, 0.42, 0.35, 0.28], "frequencies": [0.05, 0.08, 0.11, 0.14, 0.17, 0.2], genDir: "TopDown", rockGroup: "Metamorphic" }, { blockcode: "rock-slate", amplitudes: [2.9, 1.6, 0.75, 0.43], thresholds: [1.1, 0.6, 0.3, 0.2], frequencies: [0.05, 0.1, 0.2, 0.4], genDir: "TopDown", rockGroup: "Metamorphic" }, { blockcode: "rock-phyllite", amplitudes: [2.9, 1.6, 0.75, 0.43], thresholds: [0.8, 0.4, 0.2, 0.15], frequencies: [0.05, 0.1, 0.2, 0.4], genDir: "BottomUp", rockGroup: "Metamorphic" }, { blockcode: "rock-andesite", amplitudes: [2.9, 1.6, 0.75, 0.43], thresholds: [1.1, 0.6, 0.3, 0.2], frequencies: [0.05, 0.1, 0.2, 0.4], genDir: "BottomUp", rockGroup: "Igneous" }, { blockcode: "rock-peridotite", amplitudes: [2.9, 1.6, 0.75, 0.43], thresholds: [1.1, 0.6, 0.3, 0.2], frequencies: [0.05, 0.1, 0.2, 0.4], genDir: "BottomUp", rockGroup: "Igneous" }, { blockcode: "rock-basalt", amplitudes: [10, 5, 2.5, 1.25], thresholds: [9, 4, 2, 1], frequencies: [0.05, 0.1, 0.2, 0.4], genDir: "BottomUp", rockGroup: "Igneous" }, { blockcode: "rock-granite", amplitudes: [2.9, 1.6, 0.75, 0.43], thresholds: [1.75, 1.5, 1.25, 1], frequencies: [0.05, 0.1, 0.2, 0.4], genDir: "BottomUp", rockGroup: "Igneous" } ] } This file can also be changed on a server, and I think the clients shouldn't need to change anything.
If you play with large oceans, you might still want to be even more aggressive with chalk and limestone, by lowering the thresholds and frequencies further, increasing the amplitudes, and maybe adding even more variables in each category, but I don't know if there is any limit on that, or anything else.
 
By the way, to test the differences, the best way that I know is to create a new world, with a specific seed, then type /gm 2, press F3 twice to fly with no-clip, and F2 a few times to be able to fly around faster than world generation, this way it's easy to see the soil composition. Then you can change this file, and create a new world with the same seed to repeat the process.

I figured it out!  Haha!
If I sit on the ground my character can spin spin spin, but, if I sit on the -edge- of a cube, my legs hang down in front of me and then I cannot spin my camera.
Mystery solved!