Hard to go wrong with Sanaa. As others have said, if you get there when the restaurant first opens you will have a good chance at a window seat. Also, if you are not in a hurry, you can wait for a window seat to open up if all of the are taken when you get there. We have done that a couple of times. We told them we prefer to wait for a window. We never had to wait long, and we had nice places to sit and watch the animals while we waited.
Lunch does have a few less expensive options. Sanaa is one of the few restaurants at WDW that still has a different lunch and dinner menu. Check out the menu for both. Dinner offers some nice entrees that are not offered at lunch, while lunch offers less expensive sandwiches, as well as a couple of the more expensive dinner entrees (those cost the same as they do at dinner).
My main concern in your situation would be having to leave AK at a certain time. I like to take my time at AK and there is so much to see and do- especially with Pandora. The animal trails are nice in the early evening, too, and you would miss your evening hours at AK if you had to leave to eat. You might be doing something you enjoy and have to leave it so you can hurry back to make a reservation. That is no fun. Sounds like a good day to sleep in, have an early lunch at Sanaa, and then have time to explore AK for the rest of the day.
