Hi iKure,
It sounds like you picked your comfort level well 
I’m not quite sure what you mean by the “metal types” of foundation. If you mean the 14" type like this … then it should fit inside your bedframe with the slats and center rail removed. If you mean a metal grid foundation like this or a slatted wood foundation like this, then it would go on top of the slats in your bed and you would also need to make sure that the center rail in the bed had good support to the floor to make sure it didn’t sag. If your bed already has slats that are strong enough and close enough together to support the mattress like this then you wouldn’t need a foundation at all except if you needed the extra height. You can also attach a headbord and footboard to the 14" metal platform frame as well. Your mattress will generally do better on a firm and non flexing foundation that a box spring with springs in it which are usually (but not always) used for innerspring mattresses.
Post #2 here and post #29 here may help you track down where the heat is coming from. Since your mattress has wool in the quilting and a cotton/viscose cover it’s not likely to be part of the problem which means that it could be the mattress protector you are using (the thinner waterproof kind can sleep warmer than the other types mattress protector because they aren’t as breathable as the ones that are only water resistant or moisture absorbing), your sheets (polyester is warmer than natural fibers), anything else that you are using on top of the mattress, or the bedding (your comforter). It could also partly be the softness of the mattress because the deeper you sink into a foam layer the warmer it can be. It could also be a combination of all of these but as you suggested the first thing I would check is the down/feather comforter.
Thanks for your feedback and report
Phoenix