As in it's not a scam, it's also common for products past as little as 2 years end up with no support (nod to "Smart TVs"). And while I'd love to be disproved, another HP tech will likely repeat your first experience.

If Costco doesn't take them back, I consider Ebay to be the next best thing.

WHATEVER YOU DO, DO NOT FALL FOR FAKE OR SCAM SUPPORT SITES.

https://www.google.com/search?client=opera&q=OfficeJet+8600+"SOLVED"+Regional+Reset finds others were able to do a regional reset but your issue sounds different as in you want to use cartridges that are not for this model or such.