I have a plan to improve the ledge and bought some top soil to mix with the last of the soil conditioner to bulk up the soil levels a bit before sowing more seeds there.
After a lot of musing, I decided where the remaining perennials can go and concluded that yes, the old plum tree is dead. Now to see about removing it.
I put copper collars around the sweet peas that have come up. I planted 4 more perennials, which also involved first moving 3 other live plants that are low-growing plugs from the back of the bed to the front. One entirely dead remnant of an unknown plant came up too, bringing with it a big cube of dry compost.