It looks like you have done your share of work to inform the adcoms that you are interested and an enthusiastic applicant. I am not sure about an additional LOR. For most schools I know, having an additional LOR would do no good. I would only suggest letter of interest and update grades but you have already done that. But I really don't think that at this point it hurts to get one. But the best strategy is to ask the school itself or perhaps if you dont want to be that direct with the school, ask your undergrad health professions office. If you are not an undergrad pm me and I will ask my HPO office for you. But I dont see why it would hurt to have an additional LOR. In terms of content, I have always been told that you want the LOR to be precise, to sound enthusiastic, to highlight all the positive qualities beyond what the adcoms have already gathered from your application, to be able to show them your passion in medicine through specific examples, and of course bottom line, as my health professions advisor puts it, to be one LOR that the adcom will remember. I wish you the best of luck. Hang in there and hopefully you will hear some good news soon.