(they are in the Royal Conservatory program)

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Congrats!

a natural keyboard action

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You mean like a fully piano weighted controller keyboard, because that's what they're training/trained on? Or just a "good", non-weighted action?

— and obviously also something with a driver for OSX

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Mmm, not sure "driver" really applies here. If it's MIDI, it'll be handled by a MIDI app, and MIDI doesn't care (or know) what OS it's dealing with. You mean does it have a native Mac editor for its internal sounds, if it has any?

But what would be the best music application

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"Music application" covers a lot of ground. Depends what you mean by that. What do you want it to do?

Sorry to answer your questions with more questions! Just wanna hone in a bit tighter on the details.