I ~think~ the most time efficient could be bus to DHS and then walk/Skyliner/boat. Today for example both Epcot and DHS open at 9, with 8:30 EE. If DHS bus from CR started running around 7(?) and you got on first one, you’re 10-15 to DHS stop and then about 20 mins to International Gateway depending on how you choose to go.
Alternative would be walk from CR to MK, go through security, and then bus to Riviera, but between the security step and not knowing how return buses to resorts work that early, I think it’d be a bit of a dice roll.
Rideshare to Swan/Dolphin does work, but boat or walk from there to IG can take 15-20 minutes as well.