The wrong people to ask for information regarding treatment of disease such as ich are people who work at a local fish store.
It's like asking the Toyota car salesman "I have all this money, should I buy a Honda or Toyota?"
They sell products, livestock, and equipment for a living. One question you throw at them their intention is to drive you to the counter and pay for something, regardless if it works or not.
Learn the stages of c. irritans. Then learn the proper way to hypo. That's it. It's the cheapest way. It's just changing saltwater with filtered or RO
water. That's it. Until the salinity goes down. Then do a lot of observation and a lot of waiting.
No needs to mess with chemicals, this and that copper, this and that Kickich, those things are manufactured for people who "they" (the manufacturers) know doesn't have patience. And what happens when you lose your patience? You lose your specimen. Then you won't enjoy the hobby any longer.
Hyposalinity is the "other" Rite of Passage (in my opinion) besides water cycling. Once you erradicate ich, I guarantee you that you will enjoy your hobby much better (and become more cautious not to add fish without QTing it because it will remind you of the hard work and stress you had to go through).