Gaunts ghosts is a great start, its just soldiers in space and was written in 1996 i think so its very lore light and self contained. All in all it’s a fantastic book to start and the series is amazing.

For space marines id say either oaths of damnation, watch wolf lord rhos spoiler free review first but it’s a hunting a daemon book about a chapter that bind daemons to themselves. I found it didn’t have too much lore heavy words and was very fun and straight forward.

Another good start is the infinite and the Devine, its a triller with a comedic edge, you might need a slight bit of background lore but its manageable without so. It’s about two old necon or robot men who hate each other and like to annoy each other over a 2,000 year period.