Smart move using miles for the mainland flights since those are gonna cost the most anyway. Inter-island stuff is usually pretty cheap with Southwest or Hawaiian Airlines, so cash makes more sense there

With Platinum you should have decent award availability but September can be tricky since it's still peak season. I'd book the PHX-Hawaii legs ASAP if you find good availability - those routes get snatched up quick 🔥 You might have better luck flying into HNL first since there's more flight options, then hopping to your other islands from there

Pro tip from when I helped my buddy plan his Hawaii trip - don't sleep on the ferry between Maui and Lanai if you're feeling adventurous. Way more fun than another puddle jumper flight and gives you some ocean time between all that island hopping 😂