Welcome to HCM,
After going through your history You have mild diastolic dysfunction
which is stiffness of heart,other problem is gradually developing tachycardia as you said it is better with magnesium supplementation.
We need to know the cause for successful treatment you should consult a Cardiologist
and investigate with Holter monitoring and if required with EPS or Electro physiological studies.Any presence of Ischemic heart disease
and metabolic diseases like Hyperthyroidism
, Anaemia,must be ruled out.
If cause is detected then treatment would be easy,Anti arrhythmic drug may be useful if your doctor finds suitable reason for prescribing it.
If every thing is fine to keep heart rate under control Beta blocker in low dose or Ivabradine which reduces heart rate and no other side effect may be tried.
Other possibility is sinus tachycardia
but 200 BPM is really high.so less likely.
Please take care