While it's nice you are putting in the effort to make the ultimate system, I don't see the point when the crappy old relay just works, just provide the control voltage?
I'm guilty of just using wire, and a switch, no relay.Next time I would use a non-latching switch and a relay (as I have very poor memory and often left the plugs on the whole time I drove and after I got out).
That's a remarkably foolish way to operate the glow plugs. NEVER wire glow plugs to a latching switch, ever. That's a fine way to burn out all four glow plugs in short order along with ruining your battery. Did you at least fuse the circuit? If not, then that's a great way to burn up your car.