Ok, me no speak Nipponese... so no understand.... either the dialogue or the titles... which say what?
Otherwise, yes, it's really rare for a story to follow teens from high school to the 'afterlife'. which makes me think of K-On.... I think the magma does take them to college... but nothing in anime form to follow... and that movie was rather weak... like some OVA thrown together (storywise) in a few moments... not well thought out... easy to fix though... utilize the classic dream sequence, in which more such movies could be made... taking their day dreams to other countries... see, I thought it out!
And the same emotional stuff might happen in the 'afterlife', be it college, work etc... but it's more of the same... just a question of how much is needed before we learn our 'lessons'.... like Neo in the Matrix, to use a film example.
Add in that most high school band members don't continue to take 'band' in college... at least I think that's the case, as in competitions, most of them learn they aren't that good and lose interest to varying degrees... and might never pick the instrument up again... especially as most instruments are made for collective/group performance... not like the more popular 'solo' instruments, like piano, violin, guitar etc... which can be used just for self-enjoyment... not many are gonna pull out their flute, oboe, trombone etc. just for 'fun', which is why those instruments get donated to the high schools etc.
Now... I see it's a novel series, but no new ones since last fall 2017... and the title of movie to be released in a few months next year does seem a finale, as it's even in the title, just looked it up on Wiki:
Sound! Euphonium the Movie: Oath's Finale
It would be interesting to see the next stage after high school, and into the 'after life'... it's rarely done if I remember correctly, sometimes an OVA.