81 to 77 to 26 to 95 is probably my preferred route to Florida. (I-77 ends at Columbia.) I-81 is not quite so built up as I-95 but has reasonably frequent services like gas stations. There's no lack of traffic, but I think a bit less than on I-95 overall and it generally keeps moving pretty decently.
There is one fairly long grade you'll descend on 77 soon after it leave I-81. It's not super steep and is perfectly doable if you use proper mountain driving techniques (i.e. downshift appropriately). Probably more of a concern is that at times it can get quite foggy and that occasionally leads to crashes. They do have electronic speed limit signs to permit adjustment for weather conditions, and of course simply paying attention to what's ahead is the best defense against disaster.