So she also realized he had time leaped back with her in the second timeline. She realized he had memories of confessing to her (badly) in the first timeline. That's why she wrote a reply in the letter to a confession he never made in this second timeline. I already expected there was no saving Rin in this manga, so her fate was expected. Her realizing and replying to his confession in a final letter, was not.
Talk about unrequited love becoming requited after death. That's kinda new to me.
Not bad, but the story could've used a few more loops showing what happens if he confesses early, or what if she confesses early to him instead, etc. Especially explaining that time she said "nobody is going to move forward" when it seemed they were falling in love early. Better if it was confirmed that not only did she had memories of the first timeline, but of others too.
Show, don't tell.