Can't help much with your flow situation. Seems like you have the right idea now so all that's left is to kinda mess with it till you get it the way you like it. If you do happen to add another power head i might suggest putting it lower in the tank and bouncing it off the glass a couple times. Having flow over your sand bed (without blowing sand everywhere obviously) helps to keep cyano
and diatoms from growing on your sand. Won't stop them from happening but just makes it harder for it to grow. Just a thought though.