Oh hey, the stupid beaky helmets lol

Dark Heresy is a newer 40k TTRPG ruleset, that might help sort of.

I know it's not directly what you asked for, but Stars Without Number is a fantastic Sci-Fi RPG, and is super open like Rogue Trader was. Honestly, reading it reminded me of when I first read Rogue Trader as a kid.

The single worst thing about it is that the cover of the book doesn't look very exciting when compared to other RPGs.

FWIW, you could easily map the lore of 40k (either the fun RT lore, or the modern stuff) onto the SWN game system without breaking anything.

Why would you want to use a different system than the original RT? Let's just say that in the 30-odd years since RT came out, much better game mechanics have been developed.

Stick to Rogue Trader, its way cooler than anything GW is putting out and your kids will have TTRPG cred later. "My first game was Rogue Trader" "...what, how is that possible?"

I can't speak from experience, only observation.

GW put out a game called Inquisitor which was the closest thing to a 40K RPG they published directly. It was a 56mm game. It's been a long time since it has been supported, but it has lived on in the community under the heading Inq28 (using 28mm). https://www.reddit.com/r/Inq28/comments/182y7xi/where_are_the_rules_for_the_inq28_game/ is probably a decent place to start looking for rulesets, and https://28-mag.com/ has a lot of adjacent content.

In GW's current rulesets, Necromunda is probably their most narrativey campaign game. https://start-warhammer.com/necromunda/

If you want an actual RPG then C7 publishes two games. Imperium Maledictum and Wrath & Glory.