I do computer scripting, as well as some limited tech support for clients here on Fiverr, and a big huge red flag for me is when someone starts telling me how easy it is to do my job, or how it "shouldn't take much time". If they don't respect your skills before you have even started discussing their project, then what will they be like once you have an order from them.
Also if they "tried using ChatGPT" to do the job for them, I would seriously think twice before spending a lot of time with them. They are probably still trying to use ChatGPT, using any knowledge they can skim from you in that process. At the very least I'd go back and look through their interactions with you with a fine tooth comb to see if any of the OP's Red Flags are hiding in plain sight there.
One last red flag from me is "We have a lot more work like this coming in the future". No, no they don't. OK, maybe 1 in 10,000 actually have more work for you, the other 9,999 are just trying to get you to work for a discount.
And a bonus red flag for everyone, when you go to research the client, you see they are also a fiverr seller, who is offering the same, or nearly the same services as you. This means they overpromised on a project, and can't deliver. They are probably going to want you to do the job for a discount, or will try to get a fair amount of feature creep into the project.