Making a better caramel for granola bar

I came up with a recipe of granola bars where I melt

• 10 tbsp of brown sugar


• 4 tbsp of honey


• 4 tbsp of soy oil


• 2 tbsp of water


• Vanilla extract

and mix with 4 cups of rolled oats + 1 cup of rice crispies, put it in a pan and bake it for 20 min at 200ºC/330ºF. After cooling, I get a nice and firm granola bar for about one day, after that the bar gets soft and starts crumbling.

How can I adjust my recipe to make my bar more solid and for longer?