Thank you for the help. Our ADR is at 8:15am. I wasn’t sure about going to MK would be best since that day is Epcot day. What about leaving Polynesian to get to Epcot? I guess we could just do the bus? There is a monorail directly to Epcot, but it’s at the TTC, which is a quick walk?Taking the bus to MK is pretty much your best, and possibly only, bet given the time in the morning - Disney Springs busses won't be running generally until 11am to the resorts, so you can't use that as a hub. You can pick any of the parks, which depending on your ADR time you'd be best to pick parks with earlier openings. MK is the most convenient, because it gives you a few options to get to Poly - the monorail OR the boat. The boat will stop at Grand Floridian, then Poly...the Monorail will stop at Contemporary, TTC, then Poly. If the boat is at the dock or approaching, that will likely be your faster option - if it just left as you get there, the monorail will probably be faster.
If you use any other park as your transfer - you'd be taking your bus from Pop to that resort, then transferring to the Poly bus.
Personally, I think I'd probably walk over to TTC and take the Epcot Monorail...unless while I was walking over and passing the Poly bus stop (which is on the way to TTC) I saw the Epcot bus was sitting there or scheduled to arrive within 5 minutes. You can look at the screens at the bus stop for the arrival estimates of the bus arrival as you go by - if it's 5 minutes or so, worth waiting for the bus - if it's 15+, probably worth heading to TTC for the monorail.
8:15am is pretty early though - the resort busses typically start running from the hotels at 45 minutes before park opening...so parks opening at 9am means the busses won't start until 8:15am. Your best bet will be if there's one park on that day that has Extra Magic Hours and is opening at 8am - then you can catch the bus at 7:15am to the park, and transfer to Poly...that SHOULD get you there on time, but be on that first bus!
The other less desirable choice would be to get a taxi, uber, or minnie van which you'd have to pay for.
The walk from TTC is probably 8 minutes or so to the main building. The closest Poly buildings are about 5 minutes' walk. You could either take the Epcot monorail to TTC and walk over, or you could transfer to the Polynesian bus at Epcot. I'd probably use the same strategy when the bus dropped me off at Epcot - find out where the Polynesian bus stop is and see if the bus is there or arriving - if yes, jump on...if no, keep walking to the monorail.
If you had room to change the ADR a bit, it might give you a little more comfort room to get there - even 8:30am would make it slightly easier on Transportation.