Hello! I am also a St. Lucia 2018 bride. My current choice is Sandals Grande St. Lucia. Chose them because they seem to make the process very easy with predetermined wedding packages and decor that you can also tailor.
I took a look at Royal St. Lucia. It's located on my favorite beach (Reduit beach or The Ramp as locals call it). Royal st.lucian is much more affordable than sandals. The beach is picturesque . I chose Sandals because I wanted a private beach setting.
As a tip you should first decide whether you want t be in North or South of island. South has a lot of the tourist nature focused tours and you have the beautiful pitons. North has more night life and the city center is located there., A mall, casino, etc.