Yeah JMac steel doors and jambs are the way to go for sure and it is a code thing. The doors themselves usually have a piece of gypsum inside and the jambs are grouted with cement and perlite. The whole point is to make sure the door can be opened even if on fire.
For you guys who are looking to upgrade check out the local building salvage guys, they will usually sell you a commerical door in jamb with all the hardware for $150-$500 depending on condition. Last year I bought 3 great solid wooden doors in jambs w/hardware for $80 each. The knobs were $300 each so I feel like I did ok
But like JMac is pointing out, when you harden one point of entry you essentially soften another if that makes sense.
We have fought this fight a million times and I never really cared for any of the answers whether it be,
Expanded metal between the sheetrock and the framing (yeah, that is how they do the pharmacy and cash cage in Walmarts),
Grouted CMU Block with 3 pieces of #5 bar tied to a concrete lental (like every opening in a block wall ever)
3 staggered mats of #9 bar 8"o.c. in 18" of 5000psi concrete (Credit Union Vault)
Because fortifying shit means no more windows, no more freedom, no more natural light and cool breezes. A prison is a fortress with all the locks on the wrong side of the door.
So instead think about slowing them down and buying yourself a little time. Making them approach from your good side as it were.
Know your neighbors and let them know you.. this is the best security system in the world.