Most sources I've read suggest that your body can process at most 300 calories per hour during exercise. And a lot of sources suggest that you only attempt to replace roughly 200 calories per hour at best. You should be able to do this easily without any simple sugars. Your initial budget is much much larger than 1500 calories, you don't need to do a one for one replacement. In endurance events
your primary energy source is body fat, not muscle glycogen. You've easily got
a 10K calorie reserve.
I've done a fair number of Road Centuries and 8 hr MTB races over the last 10 years and overeating is just as bad if not worse than undereating. There are some companies/groups ( Hammer Nutrition ) that recommend avoiding simple sugars altogether and only consuming complex carbs.
There's nothing that works for everybody and everything works for somebody. The only way to know is to try. One thing I've found that works really well for me is steamed mini potatoes with a little salt and olive oil.
Eating and drinking a little bit as constantly as possible works the best. For a 6 hr ride, I'd focus more on proper hydration and a good balance of electrolytes. Unless you're at less than 5% body fat, you've got plenty of reserves for a long ride. What eating does is allow the body to use it's reserves.
Getting all this right is very tricky, most people err way too much on the side of over consumption. ( I know I've been guilty of this and it's led to some pretty terrible results. ). It's very easy to overthink and over plan all this, ( and there are certainly a lot of companies that will gladly take your money along the
What you eat or drink is generally far less important than that you eat and drink
something that agrees with you and that you like eating and drinking. You need water and electrolytes, enough calories to make them enjoyable is really all you need to worry about.
- This is just my own theory based on 35+ years of long days in the mountains, but often even just a tiny intake of food will yield a big jump in energy. I'm convinced the body "hoards" reserves until it detects an incoming supply of energy.