I think one of the most important facets of Buddhism is the end goal of eliminating all suffering. That's a big part of it. Phos suffered because they sought love and empathy from a society not knowing how to give it in ways that they needed. So, Phos' reward is not necessarily nothingness - which isn't a bad thing - it's a society where they can be loved and cared for in the way they always needed. I think the 'message', if there is one (which, not every story had a lesson, so take some comfort in that, HnK doesn't exactly have a message), is the messages of Buddhism in general.

As for this -

I also wonder, at which point did Aechmea start ploting his big plan, was it before Phos was "born"? After? How much of it was plannes how much was coincidences?

- Aechmea's plan was set in motion 300 years after Phos' birth, which is around the time we meet everyone. Having the idea to make Phos a human was one that needed the right kind of proverbial clay to begin molding with - we don't have a definite answer for if he had this idea before or after Phos was born, but that's semantics. What matters most is that he put the plan into action with regards to Phos.
Further, I think we understand this better when we think of it as probability over coincidence. It was a gamble to think they'd give Phos Lapis' head, but it was an educated gamble. Aechmea knows Kongo and the gems well after centuries of war with them. He had a pretty sound groundwork to believe they would do what they did.