I got a Serenity Bay Cabana on the 7.13 Fantasy sailing. It was not available at Silver booking, but it opened up a few weeks later and I happened to snag it! Just keep checking if you cannot get when Gold opens!
We've always been able to get a Serenity Bay cabana with Gold status. We usually sail on the Magic so less competition and I always call right when booking window opens. We did a B2B in Nov 4 years ago and got one on both legs. Keep trying and also check while on board. The first time we got a cabana was while we were still silver and on a whim, my hubby checked at the excursion desk the day before Castaway and one was available. Good luck!