Need to talk to cardiologist
I would recommend as follows:
Welcome to Ask a Doctor service!
I passed carefully through your question and would explain that considering the fact that several Holter tests have not concluded about any significant cardiac arrhythmia, indicates that nothing serious is going on.
Nevertheless, you are experiencing unpleasant episodes with weakness and syncope feeling, which need further investigation.
Coming to this point, after having excluded any structural heart disease, by means of cardiac ultrasound or better a cardiac MRI, I would recommend a loop recorder, which can help investigate for cardiac arrhythmia during a prolonged period of time, compared to the Holter monitoring.
I would also recommend checking thyroid hormone levels and blood electrolytes regularly.
As your complaints are occurring during resting, it seems that anxiety is playing an important role in all this clinical scenario.
For this reason, I would recommend practicing Yoga and meditations.
Hope you will find this answer helpful!
If you have any other questions, please don't hesitate to ask me again!
Wishing all the best,
Dr. Ilir Sharka, Cardiologist