IIRC, Shatner originally considered a 'planet of eternal youth' story, long before Insurrection did the same thing. He would later re-visit it for his Ashes of Eden novel.
The Shat was somewhat hamstrung by the studio cutting the budget for the movie and their insistence that it contain more comedy, in light of TVH's success. He had a much bigger and more dramatic vision for the movie than the studio was willing to allow. Had they allowed for the SFX and action sequences he wanted and cut down on the hokey comedy, it could have been a much different and more successful movie.
dealing with a budget is a reality of making a movie, and shouldn't excuse Shatner for TFF. The very successful and well-regarded TWOK was made on a small budget.