So sorry to read about Basil's condition. Unfortunately kidney tumors are fairly common in budgies, and prognosis is usually poor as you have been told by the vet. I have had 4 in the past few years with the same issue. Your idea of the platform is good and you could also pad the perches in the cage. You can wrap them in moleskin, with is like a flannel cloth with an adhesive on the back, usually sold for use in shoes. If you use this just watch to make sure that it does not get chewed on. Anti inflammatory meds may help relieve some of the pressure the tumor is putting on the nerves, hence causing the lameness. If Metacam is the drug you are using that is an anti inflammatory as well as a pain reliever. Hope the knotgrass arrives soon and gives him some relief.