"Bonjour madame Dupont" is respectful
"Bonjour Maëlle" is informal
"Bonjour Dupont" is rude.
"Bonjour Maëlle Dupont" or "Bonjour DUPONT Maëlle", using whatever the user put as their "name", is just neutral (though the former is less formal than the latter).
Generally, you only want to address young people by their given name, and adults are more likely to find being addressed by their given name rude, than young people are to find being addressed by their family name "weird" (unless they are maybe less than 16).
I find that using just the given name in any kind of official communication often has a "fellow kids" vibe.