The baggage discount for HA members is fairly new.
Since air is the only method of travel between the major islands for everyone, including Hawaiians, the flights between the islands are frequent. If it was a holiday or high season, I'd make intraisland reservations right away. Otherwise, you can wait a few months, but if you want the most convenient flights, you shouldn't wait too long.
For those flying to or from another island on your arrival or departure day, I'd highly recommend making your intraisland flight on the same ticket. It's often more expensive to do this, but the way airlines are changing flights and flight times, it is the least stressful way of ensuring that your flight times match up. If you book them separately, not only will you be on the hook for change fees should your airline change your arrival/departure time significantly, but you won't be able to check your luggage through to your final destination at your departure city. That'll mean you'll have to fetch your luggage from baggage claim once you land, and then go back through Ag/TSA again, a major hassle. (Ask me how I know.)
But since the OPer seems to be flying to Kauai in the middle of his trip, my recommendation is to book no later than 4 or 5 months out so that he can book the most convenient flights. Most hotels have an 11am or noon check-out (and many timeshares are 10am - ugh), while check-in isn't until 3 or 4pm, so I always try to book a 2 - 4pm flight. After check-out, we usually enjoy a nice lunch somewhere and then head to the airport. By the time we arrive and pick-up our rental car, it's time to check in to our new hotel. (However, morning and midday are the most hectic times at the airport as well. Evening flights are often cheaper and TSA lines are much shorter.)
Rental car reservations I make right away, then I keep checking to see if I can beat that price. Be sure to check out all classes of cars, too, because they will adjust the prices based on inventory. For instance, one year, a minivan was way less expensive than a mid-sized SUV. Well, okay, we can drive a minivan.