You have some beautiful ideas.
I considered [name_f]Iris[/name_f] for my own rainbow baby. I also briefly thought about [name_f]Beatrix[/name_f] and [name_f]Miriam[/name_f], all of which I see have been mentioned. I loved all of their special meanings, but in the end, we chose a first name we just simply loved, and two middles which meant a lot to me.
One more idea I have for you is [name_f]Celeste[/name_f]. I think it fits your criteria.
Good luck and best wishes for your journey to motherhood. No matter what you choose, your little one will have a name brimming with love and meaning.