"I was taking that milk of magnesia + soda water mixture. Would that be a good alternative or addition?"
No good alternative IMO. Magnesium needs to be taken in a form like citrate, malate; the body seems to utilize this better. When I finally found the right magnesium supplement (took a couple of years as all was in the oxide form!) -I noticed several benefits like my eye twitches gone, better sleep, less period pain etc.