QM 1 is fine for advanced lab. Most of the experiments you wouldn't have learned the details about in your lectures anyways, so you learn it in the class. My opinion on QM 1 is that it's difficulty is probably proportional to how good you are at math, since the math used in it is rather sophisticated compared to any other undergraduate physics class. I took calculus when I was in middle school so I'm pretty good at math, and found QM 1 very easy. However, some of my classmates who aren't the greatest at math (relative to other physics majors, most of whom are pretty good at math compared to the general Tech populace) found it very challenging. Taking modern physics after QM 1 is pretty common, actually. You don't really need any of the knowledge from modern physics in QM 1 (though some things are useful in QM 2). Having a solid background in linear algebra helps a lot (what you learn in calculus 2 isn't enough, I recommend taking abstract vector spaces or the upper division linear algebra class).
Modern physics is a joke of a class (probably the easiest physics class at Tech), so a lot of physics majors leave it off until junior or even senior year. Knowing special relativity is helpful in a handful of other classes, but generally the special relativity used in other undergraduate classes is very simple (time dilation, length contraction, relativistic momentum etc., nothing like four vectors, spacetime intervals or what have you).
The most rigorous/difficult required upper level physics class is probably statistical mechanics. There isn't any math beyond vector calculus in it, but applying the basic theory to actual problems can be very challenging and takes a lot of practice. Electrodynamics can also be very difficult (though I haven't taken it yet), depending on the professor. Apparently the professor who taught it last spring and is teaching it right now devastated a lot of people (which is why I'm waiting until senior year to take it). As far as electives go, I wouldn't know. There are too many of them. I do NOT recommend taking mathematical physics. It was a complete and utter waste of time. All it was was review of intro calculus, linear algebra, and differential equations, with some special functions and complex analysis thrown in at the very end.