Are you going to head to the parks first on any of these days? Caribbean is very close to Epcot, but depending on where you are walking to (Akershus or GG I am guessing) also can add time, plus getting through security at opening time. So yes, being at bus stop at 8 so you can catch bus, arrive, and get through security preRD(sometimes they will open 10-15 min early) so you have time to walk to your ADR is helpful. There may be a wait for buses especially if some go by that are already full(7 bus stops at CB I believe) so you could wait 15 min. for a bus to come, or could be there as you walk up, so you have to be prepared for a wait just in case. If MK meal is at 10:00, you might want to try to be there before RD as well, to go in do something before meal(MK is a longer ride than Epcot). If they open a few minutes early, then you could head to one of the rides that get busy like PP or 7DMT for a ride before you eat, or enjoy Main St and take some pics. As for Poly meal, if you are not headed to park beforehand, you need to bus to MK then get monorail to Poly unless you want to Uber/taxi there instead. If you get to any ADRs early, you can always try to check in and get started.