I treated my big plastic planters like mini hugels last spring:
wood and compost underneath, garden soil/potting soil mix on top. That year I grew tomatoes in them in my
greenhouse. When they died, I chopped up the plants quite small and just put the pieces on top of the potting soil in the planters they came from and left them all winter. This spring there were still some fibrous pieces, but the soft leafy bits had broken down quite well. I planted new plants in them without changing the potting soil or disturbing it as much as possible; I planted two tomatoes, some chard and leafy greens, and some cucumber and pumpkins--though the pumpkins got a 2 inch layer of
chicken manure in their pots before planting. The tomatoes were twice as flavorful this year as they were last, and everything else produced quite well. In fact, everything is still producing except the cucumbers--I'm waiting for the pumpkins to turn fully orange.
So basically, I didn't touch the pots after the plants had died, but chopped and dropped the dead plants straight down and let them compost in place, and replanted in spring.