Very nice video. I've played around with some video editing in my photography. Your first fade-out to fade-in between scenes is a professional touch and I think it could have been used again in some of the scene changes. The effect of one scene dissolving into the next without a blackout looks good in some cases also. The background music is a nice touch also but it sounds sad. I would look for other music selections to see how it changes the mood. But, as I said before good work!
Since the subject of music is very subjective and in this case it's not a disaster, I'm going to leave it as it's now but I'm agree with you that the song has a bit of a sad or melancholic atmosphere.
Regarding the transitions between scenes, here I apply the minimalist rule, that is, I add only what is necessary. In this case, a fade in and fade out at the beginning and end of the video.
Actually there is only one transition between scenes in the video in 0.48, a fade to black and I added it because the music almost stops completely. The rest of the scenes change, there is no transition between them.
I don't see the need to add transitions between scenes unless the music, or rhythm of the scenes, demands it, or they add something to the video.
Thanks!!!