Yes, high fever is dangerous and why monitoring is important and hospitalization is required if the fever goes up. A thermostat is a (I am pretty sure) a recommended tool for parents to own and use during an infection. If the fever do go up there is common available medicine.

For covid it is good to keeping an eye on oxygen levels. There are not many good tools to do this, through the skin do turn blue when things start to go very bad. Oxygen capacity can go down a fair amount before there is obvious signs, especially if the patient is laying down. The recommended medicine is pure oxygen which will require a hospital visit.

Personally I am more worried about covid. It is easier to monitor the fever of the flu, the signs for trouble is more clear, and the medical guidelines for when to go to the hospital is well established. Worrying oxygen levels are silent, harder to measure, and the medicine isn't something that people usually have at home. The guidelines are not fully established and there is ongoing research studies on the effects.

> There are not many good tools to do this,

I've had a couple different SPO2 meters for a few years, they seem to work pretty reliably.

Not yet as accessible as a thermometer if you're talking about population measures, but since it sounds like this is something that concerns you personally it might be worth getting one.

Unsure why I haven't hear anyone recommend those before, but that sounds like a good idea to get if one get infected. They are a bit expensive however so I am unsure if I personally want to pay that without a positive covid test.

a lot of people have undiagnosed sleep apnea or sub-clinical sleep breathing problems that can be improved by changing their position-- a logging spo2 meter that you can wear while sleeping can be a justifiable investment on that basis.