mmxbreaks

I am really struggling to get my head around what's required.
UK citizen, double vaccinated with Pfizer (administered in the UK, not the US).
Here's my journey:
LHR-LAX. Arrive around 19:00. Layover overnight is 14hrs.
LAX-KOA. Departs 09:00.
Obviously I'm entering the US and therefore count as a domestic passenger for the second leg. But as my vaccine wasn't administered in the US, rather the UK, it seems I cannot bypass quarantine without a test?
Does this mean I:
1. Can get a PCR test in London T-48 (allowing for the day layover) to use this same test result as the T-72 for the Hawaii boarding? This wouldn't be with a 'trusted provider' as none exist in the UK.
Or:
2. Am forced into the awkward position of getting a 'trusted provider' test at LAX (NAAT/PCR with 1hr turnaround at $250) the night before the morning departure? I'm not sure this is even possible given the timings.
Or:
3. Can I enter the US in the usual way, with an antigen test, then travel onward to Hawaii as I'm vaccinated?

The official site seems to send me around in circles, which is frustrating. What to do...