Howdy J,
I'll just put it out there that there are really no "good" options for your situation. However, planting in buckets (with very rich, nutrient dense soil) and surrounding the buckets with earth or mulch strikes me as a viable option.
We have used large amounts of mulch (native forest mulch as well as wheat straw) around some young fruit trees is 1, 5 and 10 gallon pots. This has worked fine for us while we made room in growing spaces to plant them out. We also sensely pack the pots so they together have greater thermal mass.
They have gone through hot summers and intensely cold winters without noticeably detrimental effects. (my concern was primarily deeply frozen roots.)
here's a picture of that nursery space:
On thing to remember is that can only get so much productivity out of a plant in a bucket, primarily because there is so little soil for the plants to feed on. It's root mass can only get so big, which will limit top growth. And if he soil is not intensely fertile, you can easily run into deficiencies. The larger the vessel you can use, the better of the tree will be. Also, which a large pot, you can plant some supporting understory species in the pot with them. Primarily nitrogen fixing plants to help supply that often limited resource. I am fond of dutch white clover.
Including worms into these pots can also help to ensure that the nutrients in the soil are as biologically available as possible.
Hope that helps with your situation.