I need an alternative term for “master,” as in someone who has “mastered” a particular craft [duplicate]

Virtuoso could work:

A person highly skilled in music or another artistic pursuit.

It fits the fantasy flavor. Virtuoso is a rank of spell caster in The Sims 4: Realm of Magic (the level above “master”). It’s also used to describe real world magicians:

To become an actual virtuoso magician (and not someone that can just fool the public into thinking you are), you have to understand the breadth of expertise that it would require. [...] Shin Lim has probably had the most stunning rise in a short time. But he is not a virtuoso in a broad range of magic.

This doesn’t help you with a synonym for masterwork/masterpiece but you can go with a completely different term, such as tour de force:

A feat demonstrating brilliance or mastery in a field

primary or main appear the most common and appropriate (rather boringly). For the relationship you describe, with a "master list" that all others are synchronised to, the relationship may be defined as main/replica or primary/replica or primary/secondary: GitHub proposed the use of "main" while Django now uses "primary/replica".

Alternatives such as provider/consumer and source/replica are also used. (Personally I think primary is good as it indicates clearly the status as number one authoritative source, while main has too many other associations to be a clear name.)

Wikipedia's article on Master/slave technology includes a discussion. It says that "slave" is more often considered offensive than "master" but offers various alternatives for both. The BBC have also written on this.

From the OP's comment left below one answer:

someone who has completed all qualifications and training, but not necessarily the best

I suggest the word adept. The dictionary definition does not say much in the context, but it is a term used in magical societies to indicate someone who has passed their basic training and can work towards higher levels. The term was used by the Order of the Golden Dawn for their 2nd degree members.

This works for your second example