Yep, for a new player to the series who wants what you seem to, I would probably recommend Sekiro.
My personal favourite is Dark Souls 1 for its atmosphere and gameworld, but that is quite old now and Sekiro looks much nicer and plays smoother and snappier. It's much more like an action game and less an RPG. If you don't want the leveling elements it is arguably the most appropriate, since whilst you can grind levels in DS 1-3 if you want to, in Sekiro you can't grind for more power in any meaningful way becasue the most impactful upgrades are locked behind game progress rather than experience points.
It is really quite hard, though. The final boss for me was harder than anything in any of the Dark Souls games. In general I think the difficulty of the Souls games is a bit overstated, but I found the last chap in Sekiro a major challenge.