COOL! Your power problems can be resolved by powering the relay with a separate power supply. You may also need some kind of photodiode isolation to protect the pi and ensure 100% on/off functionality for all 28 relays. Are these solid-state relays? I've had the non-solid-state analog ones fail after a few thousand switches and only use solid-state now. Where did you buy it?
RE: Custom 28 Relay Controller Board for Raspberry Pi 3