Sentry is just “what if Superman was insane?” That’s basically all there is to that character.
What makes Adam a great character is that he is a Superman expy in all the ways that matter. Oh sure unlike Kal he’s not an alien, he gets his powers from anti-matter radiation if I recall correctly. But he’s got the most important common ground: Adam is a hero because he deeply cares about others well-being. He’s not doing this because his uncle died or because he was a weapons dealer, he’s not atoning for past sins or trying to work through trauma. He wants to make the world a better place because he empathizes with peoples plight but he’s stuck in the Marvel Universe, where the civilians are pieces of **** who routinely commit genocide (as opposed to DC’s civilians which on their bad days tend more towards simple paranoia and fear mongering as opposed to routine genocide), and all the “heroes” are basically adolescents, constantly fighting one another over dumb meaningless crap and committing moral atrocities that have to get swept under the rug to even allow them to continue to be seen as heroes.
So he’s a morally upright and powerful hero stuck in a universe that basically was created in deliberate contrast to the archetype he evokes. That’s a really interesting set up, reminds me of the High from Ellis’ Stormwatch. But it’s also why he doesn’t really fit with groups like the Avengers. He’s not neurotic or driven by ego, he’d never let the bull**** Tony Stark regularly pulls slide, so he’s pushed to the side because otherwise he’d lose what makes him special: Being a flat out incredibly powerful good man in a universe where those mostly don’t exist.
I do think taking him into space could work, although I don’t think the Guardians are a good fit. Out in space Adam can be used to his full potential, whereas on Earth he’s always going to be roadblocked by Marvel’s insistence on being “the world outside your window”. On an alien world though? Adam could radically shake up the status quo. I’m picturing a mini or ongoing akin to something like Star Trek where Adam goes from planet to planet helping.