You can run 8/3 UF thru the house, out and to the subpanel. If it fits thru the 3/4 inch pvc or not I'm not sure. If it does fit it will be tight and I doubt you'll be able to pull it thru even a single 90. You need a ground, but it doesn't have to be a separate green THHN ground unless its a pool subpanel. You'll be digging more (24 inches) to get the UF properly buried as opposed to the PVC (18 inches). If your just worried about the quality of the splice use a terminal block instead of a nut or run the pvc indoors all the way to the panel.