You will eventually need to finish off everything in Broken Valley (you can leave the quarry tower until later), though you can wait until you are mostly done on the island (there is a warning that that point).
Your first encounter on sentinel island you can mindread to go up a level, so the higher level you are (I made it to level 18 in the fjords) the more of an experience bonus that will be. You don't need to get to a high level, but it would be better if you got to a new level before entering, to get a full level bonus.
Welcome to the forum.
