The loop in brief
PC Gamer says that after Good Home and the first letter reward, Coen can send four more letters to Lunka—one for each Sanzhani quest completed around Vale Sangora. Sending a letter creates a new quest.
Quest Walkthrough
Letters to Lunka: Sanzhani Quest Loop
How the letter system opens follow-up tasks and information about Coen’s family.
The source explicitly advises returning to the main burrow after each completed Sanzhani task to send the next letter. It also describes a final reward after the last letter.
Spoiler-safe wording
This page should explain the loop and its family-information purpose without enumerating every location or final reward in the opening sections. Exact markers and outcomes belong behind a spoiler disclosure and must remain linked to the cited walkthrough.