The term “space opera” does not derive from musical opera, and does not have to be grandiose or operatic. The term was devised from “horse opera,” meaning an ordinary Western-genre movie, also known as an “oater.” This indicated the ordinary genre story with spacemen and spaceships, robots and blasters. Now, some of those space operas were pretty grandiose, but they weren’t required to be. What space opera meant is that the story wasn’t going to be very hard scifi, wasn’t going to wow you with new scientific speculation, but would be a good adventure in other worlds and maybe across the heart of the galaxy. The classic space opera series was E. E. Smith’s Lensman stories.
Somewhere along the line, the idea that it was common space adventures got lost, probably when fandom was trying to get away from all derogatory remarks, and made ordinary abbreviations like “scifi” into forbidden terminology.
At that time, people tried to come up with new reasons for the term, and tried to backform the definition from the few things they remembered as having been called space operas — which were the more memorable stories, not the vast majority that originally earned the name.
The Lensman series by E. E. Smith, but also his Skylark series, with privately built, almost garage-built, starships. If you know these Golden Age series, you know space opera.
The Northwest Smith short stories by C. L. Moore, where her hero adventures on habitable Mars and Venus. Said to be part of the inspiration for Indiana Jones, especially the name.
The Demon Princes series by Jack Vance:
The Star King
The Killing Machine
The Palace of Love
The Face
The Book of Dreams
Hunters of the Red Moon by Marion Zimmer Bradley and The Survivors by Marion Zimmer Bradley and Paul Zimmer
The Starship series by Mike Resnik: Mutiny, Pirate, Mercenary, Rebel, Flagship
Star Trek and all of its spin-off novels