I’m a bit late to this but this is what i would personally do.
Order a small amount of your product just to get started. Maybe 50-100 units and get it into amazon and just see what the sales are like. It wont kill you to not have stock for a little while in the best case scenario where your estimations for monthly sales are correct. Then take that data and see how much stock you’d need for a month and start looking into how to slowly start incorporating your suppliers lead time into the times when you need to order new stock and get it sent to amazon.
And from speaking to other people who have had success in business, there are cases where products that seemed to be great failed and products that they didnt suspect to do well ended up being some of their best ones. Its great that you’ve assessed the risks of your product, however you truly wont know until you try.