The vitamin d deficiency should be fixable by just supplementing, which would be a good idea even if it doesn't cause any symptoms, because it still increases the risk for things like osteoporosis and various autoimmune diseases. Don't know much of anything about Hashimoto, but it appears to be capable of causing lots of symptoms. I wouldn't be surprised if depersonalization was one of them, but I'm not aware of any research about this. But I assume that your Hashimoto is already being treated.
Yeah, I'm also a high scorer on depression scales. So far 38 points was my highest score on the second revision of the Beck Depression Inventory and some years ago I was even talking with a psychiatrist at a university clinic about trying electroconvulsive therapy.
Basically it's trial and error. Your options are also dependent on what you are actually willing to try. If your attitude on medications and neuromodulation did not change, then you are left with psychotherapy, which probably doesn't have a high chance to help or to wait that things get better by themselves.