I think you'll find that the buses getting to the parks from CBR are not a problem at all. All the complaints seem to be way overblown. CBR is pretty centrally located, so you don't have long rides to/from any park. The one situation where you may encounter problems is if you leave a park right at closing - particularly MK right after Wishes or Epcot right after Illuminations. There will be a huge line waiting for buses. If you're leaving at this time, I suggest hanging back and browsing the shops and/or having a snack for a while (45-60 minutes) before leaving the park. Other than this, you should be fine and not worry about the buses at all.
As for timeframes...I would say be at your bus stop 1 hour before you want to be at MK or AK and about 45 minutes for Epcot and DHS. It seems most people have a longer wait for the DTD/Typhoon Lagoon buses, so you could a little more time for those locations.