First of all, about the other study you are linking. It seems like a well designed study (although I didn't check how independent the study is moneywise), but this is trial registration report. So here they are saying what they are GOING to study rather than what the results are. And of course this is a study for another disease instead of covid or instead of general health. So if they would want to study what you are saying you are interested in, they should take people of the general population, give half of them the pills, the other half placebo and check what their general health will do in the upcoming months/years. What's good for someone who is sick, isn't always good for someone who is healthy. For example, many medicine have side effects, so if you don't have that specific illness and would still take them, your health would actually decline of course.
Then regarding the rest of your story. First of all, I'm sorry to hear about all of the losses and diseases in your family. I don't wish that on anyone and it is hard to go through. I can also understand how that leads you to searching for all kinds of ways in order to try to prevent any other illnesses. I guess that's a form of trying to gain back control and we all do that in different manners. It's very frightening to feel powerless in the face of illness anyway.