Holding space down (even just to activate sprinting) is hurting my hand

Your thumb is not meant for holding a key. That's not how keyboards are designed.

Look at what your fucking thumb looks like when it hits the space bar. Do you think that's a healthy way to use a finger for holding? Look at how the rest of your fingers operate the keyboard.

Others are slow, so you probably won't see this feedback again within a few years, at which point you can band-aid yourself out of this core mechanic.

Then map it to another key, genius.

Also, have your hand check by a doctor, because holding down a key with your thumb is not supposed to hurt.

Been playing piano since I was little, never had any problems

Tapping with the thumb is absolutely not an issue, and while holding a spacebar for very long times might not be ideal, unless you're sprinting through an entire act without letting the key go it shouldn't be an issue in this game either

If you're using a mechanical keyboard I would recommend getting a soft switch that doesn't require too much pressure, usually they're the red switches