That seems to be what most devs do (the ones that work them separately at least). Makes sense because it is a lot more work (writing and rendering) completing all the solo paths, and then you can probably re-use a good % of those renders later for the harem story path by just changing the dialogue accompanying those images.
The amount of new rendering and writing for the harem ending is
by definition less because you are combining/consolidating storylines as you approach the finish. And since you've already explored and mapped out their individual characters more fully in the solo routes, understanding how they would fit into a larger group would be a lot easier.
If you try to do the harem route first, you're probably going to have to take a lot of time to fully explore each LI individually first anyway to make the harem ending believeable and consistent character-wise with the later solo endings, so...why not release that individual LI content (solo paths) as you work them out and
then go back to assemble all those ready-to-use pieces for the harem ending afterward?
I play for harem endings 99% of the time, but you can't ignore the logic most devs use when finishing the solo routes first.