Post trip update:
didn't get to ride down Haleakala. I had the bike, and we drove to the top, but my wife wasn't feeling well. Didn't want to push my luck. I will say the drive up the volcano was scarier than the ride back down. maybe is has something to do with acclimating (mentally) to the ridiculous views. She was nervous about the shuttle, and wasn't that comfortable with the rental car. Three noisy kids didn't help either.
Krank cycles was great about the rental and info. they said there are no shuttles to the top. There are plenty of tour companies dropping off people to ride the road down right below the park entrance. its still another 3000 ft of elevation gain to get to the top from there. I never contacted this shuttle company, but i believe its $80 to 6500ft:
https://mauisunriders.com/
So i just ended up riding the forest, Pineapple express was a fun flow trail. Renegade to Ravine was super fun too. I would recommend climbing the East Loop Trail, the other trails are all best downhill IMO.
https://www.strava.com/activities/5729442145
my original plan was to ride Haleakala, then ride road over to the forest. Sounds like that would be around a 6 hour ride, and the climb up to the forest is a pretty brutal 1500ft.