Pulpy space opera is a bit more flexible but many sci-fi tropes could be done in a hard sci-fi context. If we take a look at the Classic Traveller A and DA modules we could see some adaptations like:
A1: The Kinunir could be adapted to boarding a ship with a mad AI computer.
A2: Research Station Gamma could be adapted, but you might need a human patron substituted for Chiree
A4: Leviathan Probably not directly adaptable but you could certainly do some sort of space exploration campaign.
A7: Broadsword - if you like military sci-fi you could run a cruiser with some marines on it (see also Aliens)
A8: Prison Planet - escape from a prison in an inhospitable location.
A11: Murder on Arcturus Station - a murder mystery on a remote outpost.
DA2: Across The Bright Face - ATV chase across a vacuum world
DA3: Death station - Investigate the space station, what went wrong?
DA4: Marooned - trekking across an alien wilderness.
DA5: Horde - If you're OK with alien critters in your 'verse.
Several of the JTAS adventures come to mind - Rescue on Ruie, Salvage on Sharmun, The Werewolf Disease, Loggerheads for example.
Hard sci-fi doesn't have to be completely dystopian or grimdark, either. Your part could be involved with a more-or-less benign polity or NGO, perhaps some sort of merchant's guild (think something like the Merchanter's alliance). You could have a culture of belter analogues that you can hide amongst. Maybe you have a home port in some neutral location. You could have something like the Falkenberg-verse Co-dominium or UN (per the Expanse) where the navy or organisation has some political clout of its own and an agenda bearing a degree of professionalism and political acumen in its own right. Holden wouldn't have gotten far without the UN's or OPA's patronage.
There can be beacons of light in your setting.
The other leaf you could take from the Expanse's playbook is that maybe your party aren't complete nobodies. Holden et. al. were in a unique situation of having a modern warship, facilities where it could be maintained and patronage from various friends in high places who had the clout (and incentive) to keep it that way.