Bonjour William !

That's an interesting question: there is a nuance between these two expressions "de bonne heure" and "en avance".
"De bonne heure" means "early" in the sense of "at an early time of day",
whereas "en avance" means "early" as in "in relation to a specific given time".

Therefore, you use "de bonne heure" with verbs like "arriver", "partir", "se coucher", "se lever"...
but it sounds a bit weird with "être" as "to be early" implies the second case (i.e. given time).

However, it would work with "être là de bonne heure" (to be *there* early) as here both meanings are possible without a clear context.

I hope that's helpful!
À bientôt !