This is a beautiful design but with the white marking, it more resembles a tovero (Tobiano x Overo). Tobiano typically has smoother outer edges and at least three legs have white whereas Overo has the choppy, almost jigsaw look to it, stays below the topline (back), and gives crazy face markings.
Then again, I typically do a 'non-typical' style of tobiano so don't take it to heart, it is just a little insight XD
Now for the genetics, since this is a seal, it would have one dominant extension (Ee) and one dominant Seal (Ata) or a double dominant, all your preference. Everything else you listed is correct, so when you do the genetics, this entry would be Ee/Ata/ZZ/StySty/ff
I will send you a cheat sheet that helped me out when I first started doing horse adopts as well.
Keep up the good work!