I've encountered a very similar situation with my Blue front Amazon. Took about 9 months before she felt she could step up and sit longer than a minute before wanting back down.
Now several years later we are best of buddies. It takes time and lots of patience.
I've had a Couple yellow napped several years back as a third parront to them. They carried a LOT of past baggage.
Every bird is unique in its personality. They are not easily broken, but never impossible. So don't think that. It takes a board of patience. They over come it, trust me. It doesn't happen overnight. Most cases with being second, third or even forth parront, it can take even a year or more on average, 18 months to come around.
Amazon's are easy to read their body language. The nips are generally not hard, but a warning like saying " not yet" , I don't trust you get type of nip.
Amazon's are how do I say, like a monkey see, monkey do
type of attitude. In other words, you have to be the example.
Food for example, if you express wow this is good type of emotions and attitude, the bird sees that get that must be good, and want some.
Other times I lean on their cage and play with their toys, and act like I really enjoy it. ( This you have to play by ear as Amazon's can be very picky about their cage and who approaches.)
But point is interact with them, sing song. Amazon's love singing.