Amazon Music Unlimited still limits the number of tracks that can be loaded into the queue. That limit is 500. There is a way, though, to load more than 500 tracks.
I had a playlist that was 689 tracks long. I wanted all those tracks in the queue. So I split that playlist into two playlists. The first had 350 tracks and the second had the rest.
I loaded the first playlist into the queue then added the second playlist using Add to End of Queue. That gave me all 689 tracks.