I'm having a good deal of difficulty picturing what "vigorous growth at the base while long branches just so so" would look like. It's also not clear, at least to me, what the lower branches "springing up" means. In any case, I have not found the growth pattern of Fantastica to differ from most rhododendrons. It has no special pruning requirements, but this variety does need, in my experience, considerable direct sun to grow compactly. For greater density you can prune after bloom in the spring and then pinch out any single dominant growth tips to encourage multiple branching. I've never found Fantastica to need any more of this than many other rhododendrons, however. A location with more light may be a good idea, but then excessive heat may become a problem.

In most cases I prefer a rhododendron to find it's own canopy. Some start out gawky, but after a few years have a nice tight canopy all by themselves.

If you add a photo, your explanation of the growth pattern will be more clear.