Could I achieve a more refined nose and better facial harmony with a 'tip plasty' alone? (Photo)

If you are going to go through the procedure I'd address all concerns. It's hard to tell from the photos but in addition to the tip I'd suggest also narrowing the nose and fixing the small dorsal hump.

Certainly focusing surgery on the tip only is very possible. I would recommend having a consultation with a plastic surgeon focused on rhinoplasty to have your nose and face assessed in person with image morphing done to show you what it might look like with a tip only surgery.