What people find "believable" or "realistic" will vary.
Within the genre of Science Fiction there is a sub-genre of Hard Science Fiction in which it's considered more important that it could actually work. Fire Upon The Deep probably wouldn't generally be considered at all Hard because it has Faster-than-Light travel, which is one of the things you generally would rule out of Hard SF because of how conservation rules in Physics work, if you can do FtL then you can do time travel (because space and time being the same kind of thing), and then what's your story?
"The Martian" a novel subsequently made into a movie is pretty realistic, turns out the setup (a storm on Mars separates Mark from the rest of the crew) is the least realistic part of that entire story.
If that's a bit too realistic, some of Liu Cixin's shorts are pretty wild while remaining down to earth about what's practical. "The Wandering Earth" for example.
Greg Egan is very Hard, but you may find you think Greg's technically possible stuff seems harder to believe than faster-than-light travel which isn't possible at all. The Clockwork Rocket couldn't happen here, but it's set in a universe with a different dimensional symmetry, the Amalgam stuff is set in this universe, but with a society far more advanced than ours.
But then if it turns out "realistic"/ "believable" counts like Star Wars style laser blasters and stuff then I dunno, lots of things, maybe you'd like Ann Leckie's Imperial Radch stuff, (or indeed Iain M Banks' Culture novels) which aren't Hard at all.
This one follows a common pattern where it has a single piece of magic (in this case FTL travel via magical portals) and everything else is reasonably hard. Another example would be pretty much anything by Peter Hamilton (also an aficionado of magical portals, sometimes with trains).
Banks also has a magical FTL portal novel, actually (The Algebraist); they're quite popular.
Cixin's name makes me curious about your scale for soft/hard. Would you consider "Three-Body Problem" to be hard sci-fi, or at least "down to Earth about what's practical"?
Mmm. It has been some time since I read Three-Body Problem and I bounced off The Dark Forest. This also means it's possible my impressions are either erroneous because I forgot something important or it's explained/ ret-conned in the subsequent books.
In Three-Body Problem as I recall there's some pretty hand-waving stuff about sophons in particular. They're basically a MacGuffin which is used to explain whatever is necessary to the plot so as to have the Trisolarans able to interfere in Earth's affairs essentially at whim yet not obviate the invasion itself. So that's not very Hard SF.
My issue with TBP is not so much the hand-waving, it's the attempts to explain things that quickly become ridiculous, because the author is clearly out of their domain of expertise. Sophons are one example - they would be more believable unexplained, but when they're explained as a circuit on the surface of an unrolled elementary particle, well...
Or sometimes it's the way something is set up in a particular way, only to be abandoned when it becomes inconvenient. For example, when they come up with a plan to set up a trap using monomolecular filament, it is specifically stated that it can only be made in that shape. But literally one page later, when the question of how to tie it to something without cutting through that something comes up, it's immediately dismissed by the same character saying that they can also make sheets of that material, so it can be used as a backstop. Either take is believable per se, but they're clearly mutually exclusive.
On the whole, the science in Three-Body Problem kinda felt like gratuitous insertion of random (and ften misunderstood) pieces of popular science into the story as a deus ex machina.
Within the genre of Science Fiction there is a sub-genre of Hard Science Fiction in which it's considered more important that it could actually work. Fire Upon The Deep probably wouldn't generally be considered at all Hard because it has Faster-than-Light travel, which is one of the things you generally would rule out of Hard SF because of how conservation rules in Physics work, if you can do FtL then you can do time travel (because space and time being the same kind of thing), and then what's your story?
"The Martian" a novel subsequently made into a movie is pretty realistic, turns out the setup (a storm on Mars separates Mark from the rest of the crew) is the least realistic part of that entire story.
If that's a bit too realistic, some of Liu Cixin's shorts are pretty wild while remaining down to earth about what's practical. "The Wandering Earth" for example.
Greg Egan is very Hard, but you may find you think Greg's technically possible stuff seems harder to believe than faster-than-light travel which isn't possible at all. The Clockwork Rocket couldn't happen here, but it's set in a universe with a different dimensional symmetry, the Amalgam stuff is set in this universe, but with a society far more advanced than ours.
But then if it turns out "realistic"/ "believable" counts like Star Wars style laser blasters and stuff then I dunno, lots of things, maybe you'd like Ann Leckie's Imperial Radch stuff, (or indeed Iain M Banks' Culture novels) which aren't Hard at all.