I haven't used RideMax in a long time because since I've obtained small children, setting a strict schedule of what to ride and at what time doesn't work the best. Last trip, I got to watch the Disney Jr. show no less than 6 times

However, when I did use it, I found it overestimated things - we got to our break before the scheduled time.
What RideMax will do is take all of your attractions, and find the lowest wait times for them - which happens to be in the mornings and evenings, not mid day. Thus, it will put your rides when the lowest predicted wait times are, which will leave midday open. I believe you can change the hours saying you are leaving early, and it will schedule stuff earlier. But be warned - your wait times for the attractions will be longer then.
So, the short answer to your question "why is there a big break" is because RideMax is all about wait times.
The more attractions you add, the more it will be forced to schedule them during the busier times. However, you could also use the breaks to do things like go to Tom Sawyer Island, ride the Mark Twain, see Mickey and the Magical Map, etc.
As far as how many you should add, just keep playing around with it. Add your favorites multiple times. Add a whole bunch and see how long it says you will be in line for some of them. Then tweak and remove/add until you find a schedule you like. And then think if you'd rather be napping or swimming in the hotel pool instead of waiting an hour and a half to ride Splash Mountain that third time
Before I had kids, we would plan our day, and use the long break time to just be spontaneous - go where we felt like, jump on a ride we were passing by, etc.