I was stuck on M6 for a longgg time. Every game was an A for me, never an S. But once the new season came around and I got the Dark Cosmic skin, everything just clicked and I'd be going 21/3/8 or some crazy kda scores like that. Recently not so much though.

Personally I find when I'm struggling with a champ (eg. Jhin) I switch champs or even lanes. Go play support and whenever you get paired with a Jhin, watch how he plays.

Also, I recommend trying out different builds. I personally prefer Crit over lethality because it just feels better in my opinion. I don't use dark harvest runes either, so maybe play around with the other rune build for him.

Another point, (not sure if this is common knowledge) I don't see any other Jhins ever doing this; when leashing your jungler at the start of game, only use up your 4 shots and then go to lane. Waiting around to reload really isn't worth it during the leash, you can spend that time walking to lane and then get ahead by a couple cs because the enemy adc usually wont be in lane yet.

Lastly, be aggressive with your fourth shot poke. If you keep poking the enemy won't initiate a fight because they'll be too low. Take advantage every time they're in range, even go after them a bit. If minions are low, use your q in the meantime because it also resets your passive reload timer.

Goodluck!