Hi firtree,
Bearing in mind that comfort is very subjective and there are many variables involved (there are more than a just a few body types and sleeping positions which can make a difference) with your much lower weight I would tend towards a latex top layer in a softer range in the area of about 19 ILD. There would be little point in adding it as a separate topper because this would turn your current top layer into more of a “transition layer” (which has more of a support function and less of a pressure relief function) and you may need firmer layers closer to your body (providing less distance for your hips to “travel” and sink down before they are “stopped” by the firmer layers).
The ideal thickness of the top layer would depend on how much time you spend on your side vs other positions and on your body shape and weight distribution. For example if you are more “curvy” and the width of your waist is much less than your hips or you have relatively broad shoulders then you may need a thicker 3" layer to “fill in” the gaps, “allow” your shoulders to sink in enough, and provide a more evenly supportive surface between you and the mattress. The “gaps” and the pressure points (hips and shoulders) are more with side sleeping than with back sleeping so side sleepers generally need thicker comfort layers. If you were not as curvy or your shoulders weren’t as wide (or you didn’t sleep on your side) … then you may do fine with a 2" top layer because you wouldn’t need to sink in as much in the lighter parts of your body before your torso or your waist came into firmer contact with the mattress. The advantage of a thicker top topper would be additional pressure relief. The advantage of a thinner top topper would be better and firmer support under the pelvis (there would be less “room” for it to sink down)
I would also bear in mind that your wool topper (which it seems you will need for better ventilation) will reduce how much the latex will compress and its ability to take on the shape of your body.
The solid wood will certainly be supportive enough and wouldn’t feel much different from solid slats although the Ikea system has flexible slats so it could be adjusted to firm up the area under the hips which could make a difference (although these tend to work better with a thinner mattress because the more foam there is the more the compression forces are absorbed before they reach the slatted foundation).
In terms of sleeping temperature … the slats on the bottom probably won’t make much difference vs a solid platform because it’s the ventilation of the upper layers that will make more difference in humidity and temperature control. The ventilation between the mattress and the foundation (in combination with other risk factors) can make a difference though with the risk of developing mold and mildew issues as well as affect the risk of a higher dust mite population (see post #10 here) although there is not general consensus about this and it depends on the other factors involved and where in the “range” of risk you are most comfortable.
Phoenix