Only you can decide how best to spend your resources. Time, money, attention - are all valuable commodities.
Amazon passed on the script. You don't know why, beyond the fact that they didn't want it. Could be crap. Could be they are already developing a project very similar to it. You don't know, and aren't likely to find out.
Will someone else want it? Who knows. It's possible.
Getting feedback on scripts is an absolute necessity. The simplest way, is to share it with other screenwriters. Share your work here, for example. Find a screenwriting group in your area. At the very least, they'll find format, spelling and continuity errors. Don't laugh at that. I finished up a script, had six people read it. Five of them got back to me with good notes. I made the changes, then sent it off to a producer. After I had mailed it, the sixth one sent me an email. "Oh, you know, on page 72 - you've got the dialog slugs swapped for those two characters, half way down the page." Crap - I sure did. It was a very important, compelling scene. I KNEW who was saying what to whom. It was obvious in context. And five of the six readers DID NOT CATCH IT. But the sixth did. Fresh eyes are really, really important. I can only hope that the producer blows through that scene, with complete understanding, and doesn't get thrown by the swapped tags.
Whether or not you should enter a particular contest, is a tough call. IN the business, only a handful carry much merit in terms of garnering a read or a request. Nicholls, of course. Austin Film Festival, Sundance - to name probably the top three. That doesn't mean others don't have value. Look, some will offer a chunk of money, and shop it around. That's nice. Sometimes, just placing in a big contest, is validation to keep writing. Made it to the semi-finals or top ten in a major festival? Congrats! When you're doubting yourself, you know that people who had to wade through a ton of schlock, thought your work was better than most of it. And they aren't married to you. Put the certificate or trophy on your shelf, and let it inspire you to improve your work.
Some contests offer feedback and notes, simply for entering. Well, that's good if the notes are good. If you spend fifty bucks entering a contest, and get two pages of notes, that make good sense - then it was worth it.
But you can also BUY notes - from professional readers. But are the readers savvy? Do they know their stuff? What are their credentials? You have to do due diligence in researching professional readers and feedback resources.
Start with the 'free' area. Try share your work here on the forum. Ask for beta readers. See if there isn't a screenwriting group in your area. Then move on to hired help. When the script is as good as you think it can be, send it out.
Understand that people will ALWAYS want changes, have suggestions, even if they love the script. ESPECIALLY if they give you money for it. That's what makes film-making so collaborative.
Start writing your next one.