The real solution in my opinion is going to be to steer away from the bullet sponge school of thought and increase the danger of zombies by utilizing more creative mechanics. I know this can take heavy development time, but that is what an alpha is for, and let's face it, this game isn't getting any younger. Truthfully, it's spent more time in early access than many games today remain relevant after release.
But, I digress. My no-nonsense solution would be to make the zombies truly unique. We have a few that stand out, such as spider zombies, and with their new long jump, it increases the danger of fighting them without needing a significant increase in HP. Now picture a looting run. You enter a POI and start scanning the room, paying close attention to corners and areas they could be hiding. You spot your first target. If it's Marlene, Joe, Darlene, Arlene, a nurse, Boe, Yo, skateboarder, businessman, or any of the other normal zombies, do you really care? No, you're going to dispatch them with your normal method. The only thing you care about is if they are feral or irradiated,or both. This is an absolute waste of potential. Each zombie could potentially have a unique strength and weakness. Cops can spit, the fat zombies have high block damage, spider zombies can scale walls and now long jump, football players can run in daytime, soldiers have high HP (presumably due to the armor), Burned zombies can ignite you, etc etc etc. Each zombie should have something that makes them unique in the way you should handle them and not just their loot tables and minor differences in HP. Nurse zombies for example can have a higher chance to cause an infection. Stands to reason since nurses are typically more exposed to diseases than most and in their undead iteration might carry more as a result. This would warrant a lot of careful thought and consideration, but it would change the game for the better since you would have to actually learn each type, what it's unique trait is, and how to deal with it. Some traits would be fairly benign unless in the right situations. some would be dangerous as hell and need to be mitigated/avoided at all costs. Something for TFP to think about that would add immense value to the game without a massive amount of development time due to the foundation already being in place. At this point, it would just be a matter of tweaking and adding some creative mechanics.