Also the whole "testing" part. Unless you're just culturing an already known virus (in which case, why do you need to synthesize anything? You have it in a test

Also the whole "testing" part. Unless you're just culturing an already known virus (in which case, why do you need to synthesize anything? You have it in a test tube or something) then there is not in fact any real way to know whether or not some particular genetic modification translates to any type of favorable property. Even gene-linked desirable traits in one species may not correspond to that trait in another, particularly when your metrics are things like "transmissivity" and "lethality" and not "I need slightly more peas per pod".