> If it’s not causing nausea and palpitations, it’s not hot enough.

This is just so wrong. I use a 110C sauna pretty much daily, and I've done very hot onsens before, and I've never got nausea. The closest I've come is feeling lightheaded, but that's only when I combine it with ice baths. If you're feeling nauseous, you probably have a poor diet or an electrolyte imbalance

Let me guess that when it comes to exercise you think that you have to experience pain or almost pass out to get optimal adaptations? I guarantee that pushing your body to that level is highly counterproductive