Since the conditions for reaction spells are much more specific than the general bonus+cantrip, then here specific trumps general
That specific rule (reaction spells are cast in response to a specific trigger which can occur on your turn or someone else's) doesn't state that it trumps the other specific rule (when you cast a bonus action spell, the only other spells you can cast on your turn are cantrips).
So which one is more specific? Let's look at what they let you do:
Reaction spells can specifically be cast on your turn your someone else's turn. (There is no such thing as "not part of a turn". Everything happens on someone's turn.)
Bonus action spells specifically limit you to cantrips on your turn.
Neither is really more specific than the other. One allows you to cast in response to a trigger, the other limits the types of spells you can cast.
Both rules are in effect: You can only cast a reaction spell after casting a bonus action spell if the reaction spell is a cantrip.
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