I don't know that I can articulate just how perfect of an ending this is for the show, particularly as an adaptation, but damn I'm going to try.
I recently reread the chapters covered by this episode in anticipation of rereading the rest of the manga after this. This sequence is only about three pages long and, for someone who knows what is coming, who knows that Frieren will meet at least some of these characters again, it didn't have much of an impact on the page.
On the screen, however, this sequence carries so much more weight. The flashback is reframed to be from Frieren's perspective, the montage is expanded to include other characters from her life—both living and dead—and, of course, the excellent EP is timed to deliver the maximum emotional impact. The entire show is filled with these kinds of changes that expand on and enhance what was in the manga, to say nothing of the beautiful animation.
The thing that stuck with me most when watching this, however, was what Himmel says: "I'm sure we'll run into each other again if we continue traveling. Tearful goodbyes aren't our style. After all, it would be embarrassing when we met again." This was not just a goodbye between characters; it was a goodbye between the show and the audience.
This, I think, is the ultimate expression of this show as an adaptation. In an ongoing manga—or three chapters into a ten-chapter volume—this kind of farewell couldn't work. As the end of a season with no guarantee of another, however, the message is made even clearer.
I very much want a second season of this show. I want to see these characters again, to see their journey continue. While I am sad to see them go now, all I can do is continue on my own and have faith that I will meet them again.
I saw someone calling Frieren a shonen anime whose theme is reflection rather than growth, and I can't get that out of my head. Honestly, Frieren sitting quietly, perfectly still, mana suppressed, for HOURS as all kinds of dangerous beasts and animals move around her unperturbed, until her prey lands on her and she catches it, is as triumphant a moment to me as any huge, loud, "go beyond your limits" shonen fight ever was. I'm burned out of loud, violent displays of power. I loved this display of power through utter stillness, a feat of skill we're given just enough information to appreciate.