There are questions about metals in kelp, but if you suspect that your thyroid is slowing at all you might research whether you want to take iodine. If you do, you should add selenium to balance it.
My view is that there is no point in taking probiotics if you regularly eat cultured foods.
I take magnesium but not potassium, but I have a small banana every day in my smoothie. I probably eat sweet potato once a week. What you want, I think, is a good balance of potassium, calcium, and magnesium. If you don't regularly eat sweet potatoes, bananas or oranges then supplementing potassium probably makes sense.