If you plan to take the bus from SSR to DHS, you need to be at your bus stop approximately 90 minutes before Early Entry (ETPE) opens that day, so that you can be on the first bus to DHS. If you're driving, you'll be admitted to the parking lot at DHS about one hour before ETPE opens, which is about when the buses begin arriving, so there isn't much advantage to either one except that the bus stops are closer to the Skyliner station, the walking path, and the boat dock than the regular guest parking is. BTW rideshare drop off is close to the bus stops.
After that, as above, walking to IG will be fastest. Taking Skyliner can mean running into a line at CBR where you transfer from the DHS line to the Epcot line. Taking the boat means stopping at Swan & Dolphin, then Yacht & Beach, then Boardwalk, before reaching Epcot. You can disembark at any of those stops and walk the rest of the way - if you want to do that, I'd disembark at Y&B.
Keep in mind that no matter how you do it, you'll be entering IG with hundreds of other guests from the Epcot and Skyliner resorts, as well as others trying to do the same thing you are from other resorts.