A 518 is 97th%ile. Approximately a 36 to us 'old score' folks. A
good score, but just that... one data point. I would argue that the GPA says much more (and will hurt much more than the MCAT will help). Having been an applicant with similarly discrepant number I understand where the OP is coming from. I already took 3 years off between med school and undergrad. That was enough. Would I have spent further time with a SMP or with more coursework? Certainly not. Should the OP risk time and money on a SMP and another round of MD applications? That's for them to decide their risk tolerance.
Being four years into a DO program now, my advice would not be any different than it was four years ago... Do the best you can and reach high... but cast a broad net and don't apply anywhere you wouldn't go. Knowing what's attainable is important. Having one data point that is "Harvard level" does not mean one is competitive for HMS. Having applied broadly, only
@shadow97 will really know what kind of response their application had and what's potentially realistic for the future.