If you wanted to refer to something that happened regularly in the past, you would use:
era for ser
estaba for estar.
Well, I don't think that you would use one over the other in order to refer to something that happened regularly in the past. You use one over the other regarding the nature of the event.
For example, you say "estar enfadado" and not "ser enfadado". Thus, to describe that someone was "regularly upset in the past"
Mi abuelo estaba siempre enfadado. Mi abuelo estaba siempre fumando
El hotel estaba siempre lleno en verano, porque era cuando todo el mundo tenía vacaciones
On the contrary, you say that someone "es amable" not "está amable", so to convey that someone "was regularly kind (when you saw him/her)":
Mi abuelo era siempre muy amable cada vez que íbamos a visitarle
Cristina era elegida "reina del baile" todos los veranos (You say someone is rey/reina del baile, not that someone estaba reina del baile)
So I think that you had a great idea considering that
fue comes from ser.
estuvo comes from estar.
That could be a good rule of thumb to tell between using one or the other. If you were going to say the same thing but using a present simple tense, would you use
ser? Then pick
fue. Would you use
estar? the pick