I have all the implements your talking about and after years of use I have developed some thoughts on what's needed and whats not.
A small disc for garden tractors because of their light weight doesn't want to go down in the ground until it's been broken up with something like a plow. Even then you may have to add weight to the disc. A digger is the same unless the soil is loose in nature. I would plow and then disc. The disc I use is 6.5 ft. wide and pulled by a 30 hp tractor just because it happens to be here. The plows used behind garden tractors are very effective. A garden tractor drag is a good tool to have and works well. I now have a pull behind tiller and like it much more than the mounted one on the CC1250. This is for two reasons, First is that the tractor pulling the tow behind is a standard gear drive and is very easy to keep it moving at a steady speed. You don't have to continually be manipulating the hydro. Second is that it's narrower and works well in the corn rows.
I usually disc in old horse manure in the fall and leave it dormant until spring. Then I will disc or dig it in the spring followed by the drag. A couple weeks later after the weeds start to germinate I will till the garden. Something I started doing a couple a years ago is to roll the garden before seeding. This makes marking the rows very easy and the Earthway planter really likes the smooth surface.