A full-zip jersey isn't overkill at all — it's actually the most versatile option and what most riders end up preferring. The zipper runs from collar to hem, so you can crack it open an inch for a little airflow on a steady climb, or unzip completely when you're working hard in warm weather. That range of adjustment is hard to beat.
The other big advantage is getting the jersey on and off. A full zip makes it easy to peel off a sweaty jersey at a rest stop or pull it on over a base layer without yanking it over your head. If you've ever struggled out of a half-zip jersey when you're hot and tired, you'll appreciate this.
The only real downsides are minor: a full zipper adds a small amount of weight and can sometimes create a slight bulge at the hem. But for most road, gravel, or commuting riders, the temperature control and convenience far outweigh those concerns.