In my experience, the softer waxes do not have longevity with their throw. A soft wax may have great initial HT as the melt pool will reach a high temperature quickly, but as you have noticed the throw diminishes significantly over the course of several hours.
Are you using a coconut blend? I don't think that coconut wax is a great stand alone wax anyway, for any application.
If you are ok with parrafin, I would suggest adding 40-50% hard paraffin wax, or switching to parrafin entirely. There is a reason why the big manufacturers use parrafin, it makes a fantastic tart!
Also consider the size of the tart, .5oz is not a lot of wax. The fragrance may truly be spent in that tiny amount of wax after 8 hours, depending on the strength and quality of your fragrance oil.
If you are married to the "all natural" concept, try blending palm wax with the coconut, 50/50, or try palm/coconut/soy at 50/30/20.
Finally, what makes your tarts "luxury"? Are you going for visual impact? Unique fragrance? Price? Novel packaging? Marketing something as luxury has many different factors, performance being only part of it. It is very likely that you may need to be flexible with your waxes to create your product line, so don't be afraid to experiment.