Is there a way to update the planting primary photo automatically?

Yep – you’ll just have to make sure that your planting or garden is marked for automatic primary photo updates.
To do this, edit your planting / garden and check that the Auto update primary photo on journal checkbox (under Photos) is selected.

Checking this box will mean that every time you add a new milestone with a photo for that planting / garden, the primary photo will be automatically replaced with the milestone photo.