I've always loved those circular gardens. :-)

The one thing you might want to consider is winter appearance. You can always hide winter bare spots on the bottom level with winter annuals, but a big plant in the center will be very visible year 'round. I have roses all along the front of my house, which means that in the winter I have bare stems all along the front of my house. Some plants have a nice branch structure or interesting bark, and so they look good even when naked. Roses are (IMO) not one of those plants! Every winter I toy with the idea of replacing some/all of the roses with evergreen shrubs because of this, but every spring I chicken out because I like the rose blooms. I am just trying to help you avoid the situation I have put myself into.

I know I am way north of you, so maybe someone closer to Lakeland will chime in, but don't you get enough cold to defoliate roses for a couple of months?