There are other sung-through or virtually sung-through musicals. Three from this century that I liked are Hamilton, Next to Normal, and Natasha, Pierre and the Great Comet of 1812 based on War and Peace (well, a part of it). I do agree that none of those would be considered operas. Candide, though, would qualify as an operetta in my book.
Perhaps the most important criteria in my choices is the integration of all parts of a musical into a single vision.* The movement from spoken text to song must feel natural. For instance, She Loves Me, an adaptation of the film The Shop Around the Corner, improves on the original, because those characters can express their feeling better in song than in spoken word. Especially set to Jerry Bock’s Hungarian tinged score. Frankly, it improves on the great Lubitsch movie.
Oddly, the one that misses the mark in this regard is Sweeney Todd where Harold Prince, the director, had one thing in mind and Sondheim had another. But it still works.
See also Sung-through - Wikipedia
