There's no way to make it drift proof, Analog sticks will always drift, It's a side effect from use.
the only thing you can do is try to make sturdier sticks, other than that not much.
if you do Joycons you won't need audio jack support I don't think.
if you make a good prototype, you could do kickstarter, or a different crowd funding (or pre-orders and use that money for Production and what not.)
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Gamepad can be used to a button layout like ps4, but if joycon does that, there will be a lot of people who won't get used to it.Do the detachable joycons but use the PlayStation 4 Dualshock 4 controller design instead of Gamecube. That would be perfect.