You are getting those tougher zombies because you're playing on Warrior.
I play the game a normal difficulty and the worst I've seen in the forest biome is a cop. That was doing an infested quest. I haven't seen anything irradiated, plenty of feral versions of the normal zombies. But not a real feral zombie.
When I do a mission in the forest biome, I rarely have wandering zombies come in from outside and mess with me. That happens frequently in burnt biome missions. I've done one in the desert and a very strong mob consisting of a feral fat guy , two other normals, and a buzzard managed to get in the house I was in and hit me at the same time. I played the game for 4k hours too, I survived it.
I haven't seen anything in the damage progression under normal difficulty unreasonable.