Looks like some great riding. Plenty of options with clean(ish) shoulders. Not the daily life and death adventure when I lived on Kauai. You're not downwind from the volcano very often either...
https://www.google.com/...42078m/data=!3m1!1e3

Don't do the mauka belt road often but if you don't mind driving for a nice, tough ride (at elevation) head up saddle to Maunakea Recreation Area, park there and ride to your heart's content (or limit) along the massive shoulders of saddle and go for the climb up to the visitor's center for a tough challenge. Anything beyond the visitor's center is gravel, until the very top. You'll hear a lot about mana road, you'll need a gravel bike at the very least for that.

Hopefully someone will chime in on a group ride but if not, chat with Kona Bikeworks and the guys at big island running co., They have or know triathletes.

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808 > NYC > PDX > YVR
2024 Races: Taupo

Go to Bike Works in The Waikoloa Queens market and they can answer all your questions. I’ve done their Tuesday morning 8 am group rides and there are some strong local riders joining in but can vary from week to week. There’s a fast Saturday morning 7 am ride but it takes off from Kona town by the soccer fields, not sure if you’re willing to drive that far. I haven’t been to the Big Island since 2016 so that may have all changed since then.

 
Also, consider the Big-Island-based Hawaii Cycling Club:

https://www.facebook.com/groups/hawaiicyclingclub/
http://www.hawaiicyclingclub.com/

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