@nicrs20 There are two obvious possibilities that I can think of right now. One is that you have some bad cc or mod that's causing problems. I know you tested all of your items, but bad cc doesn't always reveal itself right away. One thing you can do is scroll through the launcher's list of installed content and look for any item that you didn't specifically install, as bad cc can attach itself to other downloads. (If you try to scroll through the list and the launcher crashes, that's a sure sign of bad cc.)
The other problem could be due to memory. When you said your Activity Monitor was in the green, that's not enough information. How much RAM is your computer using right before you try to launch TS3? You can purge RAM to bring this down, or (even better) you can restart your computer, but either way, if the launcher doesn't have enough memory to function properly, you'll get a crash instead of a loading screen. Another way to bring down RAM use is to disconnect your computer from the internet before you play. Even if you turn off the in-game shopping experience, the launcher will dump a hundred or so files into your FeaturedItems folder each time you try to load the game while online. (Another RAM hog is in-game memories, but you're not getting that far for this to be a problem.)
Anyway, please check your exact RAM usage right before you launch TS3, and keep the Activity Monitor open to see what happens when you press play. And then try a new save in a clean folder, save the game, quit separately, and then try to load it again. These steps will help narrow down the source of the issue. I understand that you want to keep your mods and cc, but right now, there are too many variables, and a clean folder will help narrow things down considerably.