It appears that I need to add that the wild angle of the seat in the pic is not how it has been ridden. I did try it to see if it helped. It did keep me back on the seat but the boys did some complaining. It is currently back in a more level attitude. The seat is quite comffy when I stay pushed back but I just keep migrating forward.
This is why I think it might be a posture-related problem and not a "fit" problem. Do that pelvic tilt. Strengthen your stomach muscles and your gluteus maximus. This will flatten your lower back and make you "longer" rather than humped forward on the bike.
I could be entirely wrong.