Hi
@AaLF & welcome to the forum
Right off the bat I will admit that I've never come across any studies linking potassium deficiency to weight loss resistance/insulin resistance but I would caution against taking supplements without being tested - potassium can cause issues with heart rhythm. I personally try to get mine from my diet alone.
As to the weight loss, it is perfectly natural for it to slow down somewhat after the initial loss. You may find that some weeks you lose more than others & at times you may even seem to gain a little. Keep an eye on your diet & an eye on your weight loss
trends over the longer term & I'm sure you'll see it inch down.
Best of luck.