Referring to the quality of doing the best in what one does

What is the single-word or phrase referring to the attitude or quality of someone desiring to do the best in whatever they set out to do (regardless of the end goal).

Example:

• The new employee John is neither competitive with others, nor worried about failing in a task; all he cares about is to do the best job that he can within his control, and he never feels upset. His has a ___ attitude to life and work.

His has a conscientious attitude to life and work.

conscientious (adj.)

Of or relating to conscience; performed according to conscience; well and thoroughly done; scrupulous. OED

Someone who is conscientious is very careful to do their work properly. Collins

Characterized by extreme care and great effort vocabulary.com

Conscientiousness

This describes the trait whereby the individual perceives that they have to do everything to the absolute best of their abilities. If given a target or goal, the conscientious person will leave no stone unturned in attending to the detail of the task. Terry Radford; Between Our Ears

Conscientious men and women set a high standard of responsibility for themselves. They believe that they must produce to the best of their abilities in social or culturally approved ways at all times. The conscientious person must never under perform and they must never have it easy. Ken Chapman; Personality Making the Most of It

We often think of conscientious people as those who meticulously carry out out painstaking tasks. However, in the context of grit, being conscientious means working tirelessly in the face of challenges and toward the achievement of long-term goals. Christopher Neck et al.; Management

A conscientious person may also understand that it's one thing to ask a lot of yourself, another to expect, even insist, that you always meet these high goals. J. Oldham and L. Morris; The Personality Self-Portrait (Snippet view)