Confused about electrolytes

I have been doing IF since June end and have lost 16kgs. While it is an excellent progress, it is nowhere close to my weight goal of 50kgs. So I want to do prolonged fasts. I have attempted it multiple times in the past but by day 4(around 100hr mark) is when dizziness, brain fog and muscle pains becomes too much to bear and I break the fast. I recently heard about this sub and learnt that this was due to electrolyte loss.

I tried to remedy it but I'm not able to find any reliable electrolyte substitues. Most of the electrolytes(ORS or Fast&Up) either have sugar or artificial sweetners in it. So I tried to make my own using the wiki in the sub. Unfortunately potassium salt and magnesium tablets in amazon(in my country) don't have very good reviews. The youtube channels are confusing as well. Everyone suggests a different electrolyte and the comments in the channels are not helpful too. And I'm worried that it may interfere with my thyroid medication. I tried asking the doctors and they are appalled at this method and are asking me to stop it at once, even though my health has improved so much since starting IF. So, I'm at a complete loss.

Please let me know if there are any natural alternatives. Can I drink tender coconut water(the natural one, direct from tree, not processed) or lemon water? Or are there any other natural alternatives? If yes, how much should I consume? My aim is to do a 10-day fast. So please, help me.

Thank you.