Hi buttercupbetmy,
Based on your description and on the specs of your mattress and your weight I would speculate that your comfort layer may be a little on the firm side.
What is the ILD (or density) of your 3" Dunlop layer?
Depending on how much more softness you think you may need (see the topper guidelines here) … i would be a little cautious about adding a topper that was too thick. Other than that it will really be a matter of trial and error as you add components and see how they affect the feel and performance of your mattress.
You can read a little more about wool toppers and some links to good sources of information in post #3 here.
That’s strange that your sheets don’t fit because 76" x 80" is a standard mattress size and fitted sheets (that have the right size pockets) will usually be fine within a normal +/- mattress size variance. It could be that the pockets are too shallow.
In any case … post #7 here (and the links it includes) has more information about various sheet options and sources that may be helpful or worth talking to although sheets and bedding isn’t my area of specialty and I don’t keep an extensive database of sources like I do with mattresses. Hopefully some of the other members will have some suggestions as well.
Phoenix