I feel the movies don't feel like from the same saga. I was traumatized by 28 weeks later, but 28 days later and years seemed less than a horror movie and more like an adventure one. 
My favourite is the 28 days later. I started the saga by watching Cillian Murphy and I stayed to watch the rest. 😂 You're right, 28 weeks later is the hardest.
The first one feels more like a thriller than true horror. Like a mystery movie. It feels like a completely different saga. Maybe I ruined it since I started with the second one, and went back to watch the first one. But the last also feels different than the second one. Second one feels more like a classic zombie movie, where you are in survival mode, similar to World War Z or Night of the Dead saga.
What you say makes sense, it's probably why I liked more those who have less real zombie experience. I just looked for World War Z... it will be fun to see how Brad Pitt handles zombies, I already added it to my list. 😆