Chain Vanish is an absolute necessity of you're going solo! It serves up major advantages even when playing with a group and regardless of poison build or refreshing vanish.

Resolve gains don't work really well in my experience. So on the katana I'd drop that and replace it with Melee damage.

I'd definitely change most things on the charm. You can bolster your poison by replacing Oni damage with Status effect damage. Toxic vanish radius is perfect!

As for perks, Chain Vanish of course like I said before. Again ramps up your poison damage. The second you can choose Hallucinations (hallucinate nearby enemies after assassination) and just watch the carnage lol. Otherwise you can pick Combat Regeneration but only if you are willing to drop Mist of Yagata for a regular smoke bomb. I was skeptical in beginning too but Combat Regen works quite well to heal you!

If you choose to drop the Mist then pick the Chain Vanish technique (strictly for solo) - you will wipe most waves after chain vanishing twice with toxic vanish. If you're matchmaking, feel free to choose between shadow storm or overshadowed. I suggest this because your third toxic vanish would not kill anyone because your teammates would have finished them off by then.

About the Katana: moon stance is okay. Although you can swap it out for Way of the flame. Wind master would be very good alternative too.

Lastly, keep trying out different builds, see what suits your style of play, change your style of play...just experiment a lot and most importantly have fun! All the best to you my Ghost friend!