Fastest, yet somewhat controllable would be:
Restart until France rivals you and Austria
Select France as a rival
Improve relations with Austria, he will flip friendly with +100 and mutual rival after a few months
Ally but don't marry (don't marry anyone at the start), wait for your queen to die, she is 10 years older than your king
After the queen is dead throw around some marriages to get a new, younger queen
Disinherit Enrique
Due to the steps above you will get an event that gives you a Habsburg heir who should always be 0 years old, unlike regular heirs who are somewhat random
The Wedding can fire if your ruler and Aragon's ruler are different gender or one of you has a regency
Regency is easier to get and far more consistant. Since the starting queen is in her 50s the chance of her dying are rather high, therefore you should get a younger queen before going into regency
You can also do it without the Habsburg event, but then there's the chance of the heir being older, resulting in a shorter or no regency