If JD opts out, it will be after 2020 when his salary drops by about $4mill and he might be able to get a better deal.
As for Castellanos, he’s already more expensive than Bradley. I’d rather just bring Bradley back in that case. But if the plan is to reset, I think a much cheaper outfielder will be needed. Castellanos is 27 and is probably looking at a worst case scenario of a 4 year $60mill contract. Free agency is kinder the younger the player is. Just ask Jason Heyward...