I can't help you much on the lead front, worth keeping an eye out for secondhand as new weights are eye-wateringly expensive now, whereas secondhand prices seem to have stayed fairly similar to what they were a few years ago.
On the Seaskin boots, mine are size 10 and I use XXXL Turtle fins - they are ideal for the shape of seaskin boots, and there is not a lot of difference from size 9 to size 10.
Other option if you want a lighter fin (though with the neo seaskin boots I don't suspect you will) is the Mares range in XL, The Quattros, Superchannel, Avantis etc all use the same footpocket.
Digs.