How to make a post-apocalyptic setting more 'realistic'?

Oh, I see how this is.

 

Luckily not a zero sum game.

 

(ETA: Oops - I started way way back in the thread! Three drafts in! So, you're just filling in a little backstory? Feel free to disregard this, but I'll just leave it.)


Welcome to the Forum! I am also writing an apocalyptic si-fi story, with very specific before during and after phases. I'm all about reality, but very anti-dystopia, anti-dark age. My "what happens after?" is also very specific. So, my useful input (if any) will be general, cause I'm pretty 'out there'.
What are you angling for? How they got there? Why they're still there? The catastrophy itself? Is your story about one character group, or destiny? I love to mess around with specific solutions to unusual problems, so mine is a deeper dive. If you're not detailing all three phases, I'm assuming you have a single focus on the 'after'. From your questions, it looks like you're starting cold. That means you'll be handwaving some things and detailing others.
Hmm, let's see... I don't want to influence your direction too much, but I can offer an example to start. Say, a T=0 event.

I use The Rule Of Threes:

.3 seconds to die from impact, concussion, or crushing forces.

Three minutes to die from trauma, immersion hypothermia, or suffocation

Three hours to die from harsh exposure

Three days to die from dehydration

Three weeks to die from starvation

Thirty days for ‘wildfire’ class epidemic

Three months to grow sustainable food sources before it no longer matters

Three generations to create a sustainable population cycle to overtake attrition (in my story)

So, in fifty years, if attrition is unresolved, very, very few people are left. How good or bad do you want things to be, from your story POV? Are you going for out-of-the-box, or a good ol' romp? How much of this story do you have down?
Love to hear your ideas