If you haven't check out Far Beyond The World and Minotaur hotel, both has only one route and is pretty linear like Adastra so you might enjoy them.
My personal favorites are Tennis Ace and Password, Tennis Ace is just a slice of life type VN but god does the writer know how to make good characters and dialogue, Password is more like a some what murder mystery with some fluff and romance.
Kingguard and Echo have amazing stories, tho don't expect any good romance in Echo (echo was written by the same writer as Adastra and personally I prefer echo's writing more)
There's a crap ton more so you can look them up yourself but these are my personal recommendations