I just watched this.
It was very similar to what she had done a few years early for her 8th-grade self, and we see how well that went.
It wasn't though, it was very much not the same thing to the point where it was really absurd that she'd suddenly have the insight to change the tone that way. The original video was all about expectation, "wasn't that so fun" "I'm sure you're great" "must be great to have a boyfriend". She felt bad not because of the reminder of who she was, but because of the reminder that the 2-3 years of middle school were a wasteland of disappointment. The new video comes with the realization that things might turn out in all sorts of ways "and that's ok too." It's all about comfort and support and understanding that things may still suck but there's still hope. Watching that at the end of high school might be depressing if high school didn't go well, but I don't think it would give the same reaction her middle school video did. An 18 year old also has a lot more perspective than a 13 year old.
So yeah, the ending is very much meant to be a positive note, but it has a very "adult" perspective where the rest of the film does a pretty good job of showing the perspective of an eighth grader. The whole dad talk at the bonfire is written for parents watching not for a kid listening, the confrontation of her "enemy" feels out of place, that she'd do it at all and especially with all those other students present doesn't feel believable. So while the ending is positive if taken at face value, it feels more hollow than the rest of the film, even if it holds from being straight up cheery.
Throughout the film I was thinking back to how watching this film back then would have affected me but, while I recognize it on an emotional level, the specifics, as you note, would have been nonsensical. All the technology doesn't change the human dynamics, but it changes their expression to the point that it's easy to convince yourself it would change the dynamics. "Well, if I could have had X or done Y then of course I would have made good use of the opportunity." I think I would have found it truthful and relatable.