"Lieu de travail" is the location. "Milieu de travail" is rather the general situation of someone being at work: not just the location, but also the conditions in which the workers do their tasks, the working environment, etc.

Greg already explained the difference between lieu and milieu in his answer.

For a more abstract usage as described in the updated question, I would drop lieu and milieu and just use travail or some equivalent:

1: We must end discrimination and advance equity in the workplace.

Nous devons mettre fin à la discrimination et faire progresser l’équité en entreprise.

2: Equity and the Workplace: a complicated affair.

L’équité au travail : une affaire compliquée.

Here are other examples from the comments:

3: Equity, justice and the workplace.

Équité et justice en milieu professionnel.

4: Nostalgia, happiness and the workplace.

Nostalgie, bonheur et travail or Nostalgie, bonheur et vie professionnelle.

In a colloquial context, you might also use au boulot but then, you need to find something else than équité.

The « lieu de travail » is the physical location where one works. For an office worker that would include his own office as well as the common areas he has access to (cafeteria, etc.). For a plumber that would be his clients’ homes, work sites.

The « milieu de travail » refers more to the social environment: coworkers and bosses as well as internal rules and regulations. For a plumber, that would be his clients, inspectors, contractors, etc. The « milieu des plombiers » would refer to whether plumbers are rather friendly to one another or rather competitive, if the clients are generally nice or are price-obsessed, if there is stress, etc.