I recommend using Tomato Paste instead of ketchup, just a little more honey or sugar to counter the tartness. Give the sauce a deeper color and nice, slightly different taste. Nothing wrong with ketchup, just prefer less unnecessary ingredients and most of what is in ketchup IS unnecessary. Especially that extremely high amount of sugar and high fructose corn syrup.
I have a magic trick to ease that bitterness of bitter melon without affecting its consistency just soak it in vinegar and salt for 5 to 10mins we do that when we making ENSALADANG Ampalaya Its really effective even without cooking the bittermelon