Built-in cheats work perfectly well but it's important to note that 999% only works if the normal percentage is already higher than 0. It won't allow you, for instance, to override things like the relationship requirement to have sex with high virtue characters.
What you want for that is the Geass mod. If you installed the game via the download in the OP it's possible you have the Frontier launcher, in which case you just need to keep the right button of your mouse pressed when you pick an action to force 999% no matter what, but note it also works for NPCs! If you don't have the Frontier launcher then download AAU as it has the option as well (and you can choose whether it works for NPCs too or not):
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