If an employer has need for employees to perform a specific type of manual physical work --be it 'picking' for Amazon, or building houses, or working in agriculture, or whatever-- and they pay market rates, and adhere to health-and-safety laws... what other reasonable working conditions must be met?

For example, it's difficult to build houses without exposure to inclement weather.

Regular ambulance call-outs. Timed toilet breaks. Top floors at very high temperatures in summer. Workers doing heavy lifting complaining of back pain. Mandatory overtime of 60 hours a week. Performance targets calibrated for the young and healthy meaning pregnant women get fired for underperforming. Firing people for taking sick leave, even with a doctor's note. Wasting doctors' time by forcing employees to get doctors' notes for trivial ailments. Firing workers automatically without human intervention.

All of these would be unusual in a white-collar job; the biggest complaint I have at work is when the free bean-to-cup coffee machine breaks down and only instant coffee is on offer.

Aside from wages, it sounds like you agree with the sentiment of 'minimum working conditions', just perhaps disagree on the specifics of what that means.

And I see a major challenge (logical and practical) for employers whose business requires employees to perform tasks that others find unacceptable, or "inhumane", but which are legal according to H&S standards.