More height will likely only help, but I suggest you have the clay incorporated with the introduced soil to some extent although it is a labor intensive process to thoroughly mix clay with lighter stuff. To have the soil start light near the surface and gradually become heavier will encourage the roots to penetrate deeper and reduce the chances of the tree tipping over in winds or when carrying heavy crop. You could also stake the tree with a heavy post of metal electric conduit after planting, but incorporating some clay in the upper mix will also prevent the soil from drying out as quickly.
I think you should try the drainage test to determine how high you want the mound to be. I have used even 2.5’ tall mounds to successfully grow trees in sites with nearly constant standing water. Mounds never seem to have an adverse affect, but the higher the mound the more effort required to maintain its height. Annual mulching is the means I use to do that.