Soldering to those power pads should be fine. You really should keep tabs on your current draw though. If anything, you might need an extra capacitor or two across different points on the main power rails. What may happen, depending on a few different conditions, is that if you get a hard bass hit, your pi might reboot due to lack of power.

Everything you are using is overbuilt, for the most part. Just be mindful of where the most power is being drawn and if the current source you are pulling off of is regulated or not. (Any LDOs provide clean’ish power rail for the processors and other ICs, so if the supply voltage is correct, tap before LDOs, not after, for a higher current load.)