MaxSliderIncrease EP1 1-7-9 isn't working because some of my sliders aren't showing up in-game. I tried everything, changing the Override Custom Core Sliders in Master controller to 'True' instead of 'False', and even deleting some sliders hoping it'd make room for the others. What should I do ?

With MasterController, have you also tried to change "Max sliders in CAS" to be greater than the default 100?

Alternatively, you could try XCAS core mod by CmarNYC? Just read the description carefully for conflicts and to get the right packages for your patch/game version (there's a link to older versions in the description).
I believe, you have to get the core package and the tuning one for this work. You can edit the number of max sliders in S3PE.