You’ve non-centered b_var but a_var is still centered. Maybe try both as non-centered?
BTW see here for easy toggling between centered/non-centered.
You might consider using the offset/multiplier syntax to account for how far away from zero some of your priors are too. That and/or scaling your predictor and outcome variables. Also, are you sure a normal observation-level structure is appropriate for your data? Event-time data like this are often modelled with more survival-associated structures.