You've just captured the essence of Covid, in a nice concise manner. It's not generally a highly lethal virus, but it does some strange things - sometimes in healthy younger people, that is unpredictable. That's why all the hype (in addition to the politicization by the left). I personally believe that you are touching on the truth when you ask if 'this is a designer bug?'. I don't think it was designed, per se, but was the result of gain of function mutations (most specifically to the spike protein - which defines infectivity and range), and that this created a 'funky' virus that has properties you wouldn't expect to see in a rapid evolutionary sense.
What other virii have spike proteins?