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.
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.