How to simplify this model

[CCCRonaldooo]

I wonder how to simplify this model.

Now my setting is
n_x = 100,
Region = "eastern_bering_sea",
purpose = "index2",
ObsModel = c(2,4),
FieldConfig = c("IID",0,"IID",0)

And then my variables are Lat_, Lon_i, t_i, b_i, a_i. But there are more than 300 parameters after fitting the model. I wonder how to simplify the model so that the fit results only have fewer parameters(maybe 20) but still can estimate the annual abundance values for each category.