I haven't used Ridemax in a few years, but if I understand it correctly, Ridemax basically just takes your requested time periods and rides and tries a bunch of different combinations. So it probably won't give you the very best
possible combination. If you try it enough times, you should get a few very good ones, though.
If you're getting too many random small breaks in the day, you can try to add more rides to fill up those spaces.
Ridemax says you can schedule breaks so I'm not sure why that's not working.
For the afternoon/evening plan, the reason you have that large break is probably because it's the busiest time of the day for the rides. So you may want to try to add in more rides to hopefully force it to let you do something around 3. If that doesn't work, I'd keep in mind what FP you get at 3, then just run a separate plan for the 3.5 hours it's insisting on doing a break.
Or, you can just treat it as a break and go over to Tom Sawyer's Island, see a parade, Aladdin, 3-D shows, etc.
Ridemax will sometimes make suggestions that seem baffling. I remember ages ago, it told us to ride Small World FIRST!

The reasoning being that you can get FPs for the thrill rides and ride them later, so spend the uncrowded time riding non-FP rides. Not a bad idea by any means, but it blew my mind. Still does.
Cars Land is so new, I'm not certain Ridemax understands the best strategies just yet. That said, TSMM and Mickey's Fun Wheel are a lot less busy early in the morning, so there could be some logic to skipping the Cars Land madness. I don't know about going back later, though.
I know people are reporting that Mater's has decent lines in the afternoon (30 minutes or less?) Luigi's may be best to do in the morning. For Cars Land itself, I think I would spend some time reading plans on the DISboards before I'd take Ridemax as gospel. For
Disneyland proper, Ridemax is reportedly great.
If you go to Cars Land on a day when Disney hotel guests get in early, Luigi's will probably have a line before you even set foot in the park. So maybe that's what Ridemax is thinking there.