Kaeya's Birthday Letter Analysis
While this letter was really sweet I find it incredibly interesting that this set of Archon Quests is the one that triggered this response from him. In past letters Kaeya has expressed that he misses us, but he has never openly expressed his worry for us. Especially not to the point where it took up the majority of the letter.
Even though we have fought both gods and the Abyss alike and have had many harrowing hardships/battles.
Even in last year's letter, when we were on the front lines on a literal war with the Abyss and Natlan was on the brink of collapse (something anyone keeping up with global affairs should have known), he didn't express this amount of worry over our well being.
It is possible that Varka told the Knights of all the details and didn't allow the Traveler to blow off their accomplishments or water down how dangerous the situation they found themselves in actually was, so Kaeya got a clear picture of exactly how dangerous the fight was since we don't know how dramatic Varka's retelling was compared to reality.
But I personal think there was another reason Kaeya was so upset and concerned for us after this particular set of battles.
We know Kaeya has known about the Sinners are since the beginning of the game. It is reasonable to assume he knows exactly who they are and how powerful they are. I think the reason the majority of his birthday letter to us was him worrying over us is because I believe Kaeya was terrified for us when he heard we were fighting the Sinners specifically. That is what caused him to write to us to express how worried he was for us and to tell us how he is there to stand by our side if/when we need it as well as the rest of the knights.
Anyways those are my thoughts