Increase USB 2.0 power by adding buck converter to motherboard?
Don't do this. You'll likely damage the internal 5 V supply in the USB port by 'back-feeding' it from the additional supply.

If the additional supply powers up after the motherboard one, then before it powers up, it'll act as a load on the motherboard, and possibly the USB port will be disabled by its own internal protection. Conversely, if it remains powered after the motherboard is shut down, likely the motherboard won't be happy.