A lot has to happen before a park will be closed to a resort guest, and to be quite honest, if it's that crowded

you got there waaaaay too late. They start with closing the toll booths to outside guests, then go on from there.
On the positive side--last spring break I tried something, just to see if I could do it when we knew it would be packed. I got to MK 10 min before park opening(a 9am open), went directly back to get a FP for philharmagic. I did not plan on sightseeing through the park--I saved time for that a little later. After getting the FP, I went on snow white, pooh, peter pan, aladdin, then over to haunted mansion, thunder mtn, and splash mtn. Keep in mind that most park guests were busy looking around and I went straight to the back of the park. There were no lines at any ride because people were still working their way forward. Before I went back to philharmagic, I got on the train and rode to the other side of the park and got a FP for Buzz, rode space mtn, then philharmagic. All of this
before 11am! I was really hustling, so if you can do half that with your family, you are doing great!
Just remember--don't do what most of the people do and go to any attraction without a line. If I had gone in philharmagic instead of FP, there would have been 15-20 min for lines to form at all of the other rides. Be at any of the parks at gate opening, FP for the longest/most popular attraction--even if there is no line--first. FP will say what time you can get another one. The first ones of the day for a 9am open will usually say 1005-1105 and you can get the next one at 1010. Always get the second one before going back to use the first FP. That way the clock is already ticking for the second one.
AK--FP the safari first thing in AM, enjoy other rides, FP ITTBAB go back to
safari, FP rapids(by the time that comes up it will just in time to cool off)
MGM--FP TOT, or RNR
Epcot--FP mission space then go to TT
We'll see you in a few weeks!