If there were only one supplement to take, it would be magnesium (along with vitamin D). The complete guide to the best multivitamins on the market is still available here https://blooness.com/meilleur-multi-vitamines-quoi-choisir/
4 coffees a day = ½ teaspoon of salt lost in urine. 1 hour of sport = ½ teaspoon of salt lost. In this case, you'd need 1 teaspoon full of salt just to recover the lost salt.