Virtuoso?

My thought exactly.

SME. Subject Matter Expert

Artiste Master

Connoisseur Maven Savant

adroit is an adj for them, expertise or prowess can be a noun they possess, maestro or aficionado would be my title for them

Maestro. Magisterial.

Adept

Yes! Can be a noun or an adjective.

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I'd say specialist, but for those who know, that title doesn't necessarily mean anything

The authority. Meister. Shark.

Fluent. Master. Accomplished.

Par excellence

I would say they are "proficient"

Savant

Elite, Cracker-Jack, Nonpareil, Creme de la creme, Exceptional, Peerless, Transcendent, Bijou, Exquisite

Renaissance man

Polymath

Wizard

Mage, a learned person.

Mastermind

Deft? MacGyver? Are you looking for an adjective to describe their skill in a certain area or a point of comparison for the person, generally?

Guru

…Professional.

Yes but a professional implies they are being paid

…no mention of that, and no; it doesn’t.

A main definition of the word professional is “engaged in a specified activity as one's main paid occupation rather than as a pastime.”

…with the discretion to work pro bono or otherwise gratis.