Having 20 planter boxes all clustered together got me thinking....
It sure would be nice if there was a UI that would grab all the Planter Boxes in a radius around the player and let you 'mass plant' from there. Either a "fill all" with the selected seed or individually via double clicking on the box from the UI (similar to how you select materials when crafting).
20? You are way more patient than me, I built 8 and I planned to expand until I realized how put off I was farming at all by the planting process lol. Something like this, or just equipping seed bags to run around and plant by aiming at the planter and hit action button.