I think your experience jumping in with Sekiro will really depend on where you're coming from as a gamer. If you cut your teeth on hard-ass 8/16-bit games like Ninja Gaiden, Ghouls n' Ghosts, Contra or arcade games, the difficulty won't seem that unusual. Dying multiple times until you master a section, and having to fight your way back from a checkpoint & re-earn your stuff will be something you're well experienced at.
But if you're used to Wolverine health, respawning right where you died, and encounters designed to be beaten in the first couple of tries to keep the story moving and prevent players from putting the game down, then it might just be an exercise in frustration. A lot of players seem to think they're up for it but bounce off it when they realize they might have to spend literal hours mastering a section or boss to get past it. Not everyone's got the time, energy or inclination.
My recommendation is to start with Dark Souls. Sekiro is an action/adventure game, while Dark Souls is an action rpg, so it has the advantage of levelling. If you're stuck at a certain point, eventually you'll have tried so many times that you'll get strong enough to make it through even if you take a couple of hits. Many still consider it the best constructed game From have made, and you'll appreciate what's special about Sekiro that much more if you start there.
Bloodborne is the one everyone pegs as the easiest, but it's also a lot of people's absolute favourite so you might want to save it for down the line, rather than have everything be downhill from there for you if it ends up being your favourite too.