Monarchies should skip a generation (so, go from grandparent to grandchild). That way you reliably get long periods of fresh blood, as opposed to transitioning from a 95 year old to a 70 year old.

(Alternatively, abdicate early Akihito-style)

If de-randomizing to a degree, why not go all the way?
▪️"New Monarch = Closest Heir who is greater than 18 and less than 50."

And if de-arbitrizing to a degree, why not go all the way?
▪️"New monarch = Closest Heir* who is good in age and in virtue."

*Note: we're all technically related so anyone can be Heir