If you happen to be staying on DVC points, even rented points, parking at Aulani is free. Another perk to consider when weighing the decision of whether to rent points.
I am also a proponent of renting a car for the duration for the ease and flexibility of sightseeing, but also for the ease of getting to and from the airport. After we arrived and picked up our rental, we were able to stop at Target in Kapolei to pick up some groceries on the way. By the time we got to Aulani, our room was ready and we were able to go straight there. Worked out well.
If that doesn't work for you, I see Charley's most often recommended here. I'm not sure whether there are any cheaper shared shuttles that go to Ko Olina, but if there are, I would guess they are fewer and further between than Waikiki ones given the remote location of Ko Olina. Uber/Lyft may be more prevalent now than when we were there a few years ago, but we didn't find availability to be that reliable, especially in Ko Olina. I know that Covid has also impacted Uber/Lyft availability where I live, so that may be a factor too.