Obligatory not a disc brake user but...
this is a pretty simple problem. You have 2 materials available. Organic/resin is designed for low temps and metallic/sintered is designed for high temps. If you're descending steep, curvy mountain roads, get metallic, at least for your front. If not, stick with resin.
The following is uneducated rambling, so take it with a grain of salt:
Squealing is caused by sticking and slipping. If the frequency is high enough (initially), you can't hear it. Your spokes also have a certain harmonic frequency at which they REALLY howl. The closer you get to that frequency, the louder the howl.
So why would the stick-slip frequency decrease? Either the slip is taking longer or the stick is taking longer. Or both.
Slips get longer if the pad/rotor overheats. If you're hearing squeals after hard braking in the dry with resin, you're overheating the pads. Get metallic. Although I think if you went beyond that point and properly melted the pads, the sticks would also get shorter and your brakes would get quieter... you'd just have no brakes.
Slips also get longer if the pad/rotor contact is compromised. Oil, water, etc. I think it's normal to expect squealing in the wet, especially with sintered pads, which don't like cold temps.
Sticks get longer when your pad gets up to temp and starts biting harder. This is why your sintered pad might be quiet at first but starts squealing once you get some heat in.
Sticks also get longer when you press the brake harder. So the likelihood of squeal goes up the harder you press the brakes.
Sticks and slips both get longer at slower wheel speeds. You can probably make your front brake sing by getting off your bike, grabbing the brake and pushing your bike very slowly.
I also believe that, if you want to minimize howl as much as possible, you should install rubber dampers at all of your spoke crossings. This will a) absorb a ton of energy and b) increase the harmonic frequency of the spokes (double?). But there's a slight chance you'll go from a low howl to a shrill, ear piercing squeal in the wet, similar to cars. Good luck
citation for the damper idea:
SRAM disk brakes harmonic vibration
Other crazy ideas include:
buying a bikeahead monocoque wheelset.
buying a wheelset with much stiffer spokes (carbon spokes, like lightweight, winspace etc).