This has information about story and quest design https://na.finalfantasyxiv.com/blog/003029.html

Then they have to come up with characters, so see this https://na.finalfantasyxiv.com/blog/003359.html

Then they need translating
https://na.finalfantasyxiv.com/blog/003334.html
https://na.finalfantasyxiv.com/blog/003339.html

For rewards, I remember the models take something like 6-8 months because it starts with an vague idea, then the concept art team have to try to draw it and find the best one that they came up with, then it has to be made, animated, have an icon designed, tooltips written, then translated and tested. Just for a simple reward that some people might discard afterwards. There is more information about how long it takes to make items here https://na.finalfantasyxiv.com/blog/003401.html

My other links were to events that took place after live letters, but these are from the blogs.