When the TR guys did group rides in Kona, I recall a few of them going towards Captain Cook. I’ve driven part of the IM route and personally wouldn’t bother riding on it. @Nate_Pearson had some recent rides down there on his strava so you could check that out for ideas too.
Edited to add the old thread which might help you: Kona 2018 - Rides, Meetups, Discussion, etc