Sky 3rd is an interesting game.

You're right, it's completely different from the usual Trails game - it's shorter, faily linear and focused asides from the side-stories in the Moon/Sun/Star doors. The main story does get better IMO when it's done with the introductions and it starts to focus on Kevin's backstory, but it remains a very different experience until the end.

I had very a similar opinion of the game when I first played it, but the more games of the series I played, the better my opinion of Sky 3rd becomes in retrospective. One of the issues I have with Trails is that the games are too formulaic, and it's very easy to burn out playing many of them in sequence. This type of "breather" is something I feel is sorely needed in the series, and one of the reasons I'm liking Reverie so much - it takes a lot of ideas from Sky 3rd and improves upon them.

Also, the memory doors add a ton of cool lore and worldbuilding, including what is likely the darkest section in the series.