I do not have any personal experience with ECT but I do know of two cases similar to yours. The daughter of a friend of mine (she is in her early 30's) has tried many meds, ECT, and TMS without success. What has worked best for her is Adderall at a fairly high dose, three times a day. The other example is my mother, who many years ago had severe depression and received ECT. I think it helped a little but what eventually worked best for her was Lithium which is still used today for stubborn, hard to treat depression.

Good thing that you are still seeking answers.