Why are you ruling out Amanpuri? The beach is rather good, as is the water in April - no strong currents etc (which usually isn't an issue until well into May). These pics are taken during a mix of late April/early May, mid-summer and September:
https://www.thesuitelife.com.hk/aman...-site-visit-2/
Trisara admittedly doesn't have a great beach.
Re: FS Hoi An/Danang - USD 800/night is nowhere near rates at Bawah Reserve, even with incidentals etc.
Have you looked at InterContinental Danang as an alternative?
This is rather confusing - you're basically ruling out anything with a nice beach by saying "it doesn't have a nice beach" or "too far" or "too expensive" etc and then stating that "so many amazing hotels in Southeast Asia, but almost none seem to have a nice beach."