USB cable for power delivery max voltage
The 30V means the cable is rated for 30V max, so, yes, you can apply max 30V to the cable.

When checking the very conservative AWG chart, column "Maximum amps for 
power transmission", the 28 AWG wires can carry 226 mA for those circumstances. So, in your case, either 24 AWG or 28 AWG can easily carry 50 mA.

Small current peaks are not interesting, because the current rating is based heating up the cables, and peaks will hardly contribute.