I get short of breath and feel like I’m going to faint after walking around too. For me it’s because I developed a circulatory disorder after getting COVID. The blood slowly pools in my lower body instead of being properly circulated back to my heart and upper body, thus causing shortness of breath (not enough oxygen) and lightheadedness (not enough blood to brain). The disorder is called POTS, it’s quite common to develop or worsen post-COVID, and on average it takes years to get a diagnosis because most doctors are not familiar, so I am compelled to mention it to you just in case it is helpful. You can screen yourself quite easily at home with something called the NASA lean test.
I’m very sensitive to cross contamination (celiac disease) so I have to be very selective about my supplement brands. I use Pure Encapsulations, Thorne, Xymogen. And for magnesium specifically you might be interested in Xymogen’s Optimag Neuro - it’s what I take for RLS and sleep - it’s got magnesium l-threonate, malate, and glycinate so you can see more easily if any forms work well for you.