Add a 300mW 4.7V Zener diode across RH2, anode to ground, to provide some input protection for the MCU.
Get a ratchet crimping tool to do insulated spade connectors for the relay terminals, as soldering heavier wires to them typically FUBARs the relay because the plastic base usually isn't very heat resistant, so the contacts go out of alignment. You've probably got away with soldering the coil wires, but I'd still scrape off the solder and re-do them with crimp terminals: Auto relays go bad more often than one would like, so you might as well plan for future replacement.