Thanks for your report xracer - very useful, objective view which I appreciate.
Although I haven't stayed at a Six Senses property before, I have stayed at the Soneva property in Thailand at the time when they were both part of the same group. Whilst there I found the standard of the F&B offering amongst the best I had experienced and they also got most other things right. As you say, the "rustic" feel which I think that they overplay themselves if I'm honest, is probably not entirely accurate because it is rustic with clean lines and a modern twist as you suggest. I think a lot of people don't like the open air bathroom concept which is understandable but that we didn't find to be that bad during our stay.
Given that Soneva is/was meant to be the premium brand of the group prior to their splitting and you liked some of the qualities at the six senses, I would suggest you keep an eye on Soneva developments as they may suit your tastes.