CheapMom.
We have gone in both the summer and at the slowest time of the year (first 2 weeks of December). If you are not into crowds, get to the park at least 15 minutes before it opens. I always plan to get there 30 minutes before- which results in us getting there 15 minutes before because we almost always run a little late. The truth is that most people can't/won't get up early. Even in June, I ALWAYS have RnRC, Great Movie Ride, Little Mermaid and ToT (at 10 with FP) done before most people arrive at 10 AM or so. Once these are done- the day is a breeze

I used MGM as an example because the OP did, but it works in every park. Note the time the OP got to the first ride.
If you are onsite and have EMH- use it. We typically tour from opening until lunch time, take a 3-4 hour break for a swim and a nap, then hop to a non EMH park for the evening. In the summer, we go to the water parks about 4 hours (sometimes 4 1/2-5) before closing. Most people arrive at opening, stay until 2 or 3- then leave. The water parks are much less crowded in the late afternoon. If we get a storm while there- we wait it out and have the park to ourselves after it passes. We usually do a water park after a late night when we plan to sleep in.
I have been asked about the long breaks before, so since I mentioned it I will address it here, too. By that time of day (lunch) I have all the major attractions done with no wait in line. I don't need to be in the park any longer, so we leave to stay out of the heat- which is going to sap the strength of the those in the parks. While 'they' are waiting in lines, we are sleeping in an air conditioned room and wake up refreshed for the evening.
The crowds aren't bad if you go early because there are no crowds. Will you have company at that time of day? Sure you will

But nothing compared to the shoulder to shoulder crowds at the parks in the middle of the day.
Each person has to decide if they can do the early morning routine. I don't like fighting through crowds, so I get up

Truth is, by day two or three of the vacation I am probably more rested than those that arrived with me (even though they slept in) because I was asleep during the hottest part of the day
Hope this helps you, and you won't worry as much
Blessings,
mic_key_mouse