Problems with STAN sampling in a brm model (brms package)

Question

I am new to STAN and Bayesian modeling, and am trying to fit a model using the brms package using the COM-poisson distribution for a some count data that is under dispersed. However I am struggling with STAN diverging and crashing a lot. warnings include

Divergent transistions after warmup
fit contains no samples 
Bulk ESS low
Tail ESS low 
Largest Rhat was 4 indicating chains have not mixed

data : Dataset

My model is

brm(EstAgeInYrs ~ poly(KnwnAgeInYrs,3) + (1|ReaderID),  
              cores = 4, iter = 1000
              data = DataHI, family = "com_poisson")

I arrived at this model from testing various models using glmmTMB.

Any suggestions for making progress on this? I have identified that the first age = 1 and age = 2 var / mean > 1, and I know brm( ... family = com_poisson) struggles with over dispersion, but the rest of the data is significantly under dispersed ( <0.5).

Any help would be greatly appreciated