As an unregulated over the counter supplement, who knows what Sambucol does or doesn't do, it hasn't been tested by the FDA.
Most colds are viral and that's why your frequent antibiotic courses aren't helping.
It's great that your CXR is clear and showing nothing.
Even if you stopped swimming, regular exercise can boost the immune system and make you stronger. Try regular walks and increase your activity from there.
Good luck!
Maybe from the change of lifestyle your immunie system is a bit disoriented. If you really have nothing serious as it seems like, I can suggest you to take Sambucol which helps boost the immune system. It's made from balck elderberry which has more natural antioxidants than blackberries or cranberries. I use to get a cold relatively easy and since I have been using Sambucol I have not been sick at all. I hope it will help you.