Is it okay to sleep through a 10-minute long tachycardia episode during a rem sleep period as reported by smartwatch? does arrhythmia during rem increase risk for sudden cardiac death in sleep?

You may have a REM sleep disorder and this could be a sign of future Parkinson’s disease or Lewy Body Disease. You might be a candidate for an overnight sleep study

If your heart rate was under 120 at the time I doubt you had a significant arrhythmia but had simple sinus tachycardia seen during excitement. This could occur during REM sleep. I wouldn't worry about sudden death from sinus tachycardia in your case.