Honestly, this is the most beautiful description of Sunny I have ever seen. You are a very talented writer, my friend.
Anyway, you’re right about her kindness and selflessness. In this way she is definitely like Fluttershy, which is likely a reason why I myself like her so much (Fluttershy is my favorite G4 character as well as my favorite overall MLP character). I think this kindness and selflessness will be great for her in the long run.
Knowing this, I can say that Sunny could make for a very different kind of Alicorn compared to Twilight, a character that she is frequently compared to despite having a very different personality than Twilight. Putting the lives of others over herself would likely mean that her royal palace would be open to everyone, and she would spend every day trying to help other ponies out. Honestly, this would result in her being highly respected in the long run.
In the near term, there is another thing that Sunny’s kindness could help with: the mystery Alicorn. Honestly, while this may not be the case, I do feel like Sunny will do whatever she can to try and reform that character, even if said character doesn’t want to be reformed. If she succeeds, then it further cements her role as a leader, one that leads with kindness instead of intimidation. It would help to build up both Sunny and the other Mane Five ponies, at the very least, showing more of their traits and also showing why they work so well as a team.
Can’t wait to see more of this, both in Tell your Tale and the upcoming Make your Mark series!