Thanks, everyone. I'll likely check out the insulated steel doors available.
Fiberglass could be an option, but I'm concerned about potential warping in Phoenix climate. The old door was installed shortly after the house was completed in 1993 and has lasted a long time—it's a steel-core door with wood surrounding it? (not sure the benefit). However, after over 30 years, the wood on the brick molding and the bottom of the door is rotting.
Not sure if we installed it wrong after the house was built but I guess 30+ years is not bad for a door that isn't shielded at all from rain. Considering I just got the highest insulated garage doors on the market... this is probably the next biggest air leaker too.