Now that i have taken some time to take in the ending, I will talk in detail about what it felt to me.
As we have already heard from everyone who played the game, the story is a disjointed mess from start and won't start making sense until closer to the ending. There is a lot of content that feels like it was cut and some important information can be found through the lore scattered in the world. That said, the ending packs a really powerful punch and it did that for me, making it one of the most memorable FF ending. The story takes some massive leap of logic to get to the end but when we are there, we experience it first hand, it becomes clear what the game is all about. It is about the journey of Prince Noctis and his exact opposite persona is Ardyn. Once a notable king who was accused and banished to the darknesa. It parallels to Noctis who is the rightful king that is fighting hard to reclaim his throne from him.
As for Luna x Noctis, again it is a huge missed opportunity that their relationship isn't conveyed in a proper way but saying that Luna was just doing everything for Noctis is completely wrong and does disservice to her character. We learn that Luna and Noctis have been childhood friends since growing up and he is engaged and set to marry Luna. It is clear that they both share a very beautiful bond of love between each other and this is seen in glimpses during the cutscenes and the way they communicated with each other was cute, I might add. So while it is established that Luna really holds Noctis dear to his heart despite her brother telling her multiple times that Noctis is not worthy to be a king, she has full faith in him. But this is just one side to Luna and the other side that has an equally important role in her sacrifice is her destiny as the Oracle to the people. To see what this really means to her, it is highly recommended to see the cutscene that is missable in the beginning of the game. It might be short but it shows a side of Luna that is missed during the main narrative and makes much of an impact during her big final speech in Altissia. So it is the culimination of these two points that she sacrifices her life to save Noctis and the people of Altissia, because it is her duty as an Oracle to protect them and also her love for Noctis.
Coming back to Noctis, he has to reclaim his throne but it is pretty evident right from the start that he is not prepared for it. So he freaks out once he learns that his father never told him that he would have to carry such a big task one day. He also has lack of confidence which is very apparent in the beginning few moments when he has to rely on his brotherhood friends. The thing is, as Noctis slowly begins to travel through his journey, he is continuously mentored by his friends like Gladious or Cor and Cid, who tell him clearly that it is his destiny and purpose to bring back the Kingdom of Lucis. By the end of the game, he pretty much has made his mind and decision so when he learn that he has to sacrifice himself to bring order and light back to the world, he accepts his fate and sets out to carry it leading into the ending clash. He knows he will die since it was his fate and the same was the case for Luna. The way it ended was that both fulfilled their role and were granted the chance to be together in the after life, beautifully conveyed by the whole ending scenario. Noctis no longer remains a sissy prince but mans up and reclaims his throne and willfully sacrifice his life to restore order to the world.
At the end of the day, Noctis x Luna both had a tragic ending and both knew it beforehand and accepted it. There was just no way for the game to end in a happy way and I am quite confident in saying this is one of my favorite FF ending.