Mathematical modelling of resources vs population vs wealth generation and other factors

Question

I was experimenting with ways to model population of some country given its initial population, initial resources and some coefficients to have a control on the population and resources, and I have coded a simulation for it:

popstep = [100000]
restep = [5000]

k = 100      #Rate of consumption of resource for 1 reproduction
r = 0.0015   #Coefficient of Growth of Population
d = 0.0090   #Coefficient of Decline of Population

for t in range(1000):
    
    timestep.append(t)
    
    dP, dR = 0, 0
    if restep[t] <= 1200:  #When resources reach 1200, decline begins!
        k = 55
        nB = 1
        dP = -1*d*popstep[t] + nB*k
        dR = -nB*k 

    else:    
        k = 10
        nB = random.randint(0,2) #At every timestep, there is a choice of either no child, one child or two children

        dP = r*popstep[t] + nB
        dR = -nB*k
     
    popstep.append(popstep[t]+dP)
    restep.append(restep[t]+dR)

The output for the above simulation turned out to be:

However, I want some natural decline in population, unlike the sharp decline. What am I missing in my equation to achieve that? I would also like to know how I could make my population generate resources such that it never really runs out of resources. How should I implement wealth generation, which would correlate with the population and resources I have?

I'm doing this project for a simulator game I'm working on.

Do you suggest any reading I need to do to model such simulations?