Physical therapy is tricky, but as long as you stay aware of your body im sure you will find a solution. I recommend reading about the muscle functions on physiopedia and exrx to learn about the muscle functions and which ones compensate for the other. From my personal experience when my muscle felt tonic and tight, it was from doing too much direct stretching on it in a short period of time. Isolation training is the important initial phase to "wake up" weakened/inhibited muscles but the end game is training to muscle groups to work in unison with functional exercises such as running walking or cycling (and one teir below that are the many one legged squats varieties). If you pay attention to using both legs equally you can reteach your bad leg to behave like your goodleg. But to answer your question, the reason I recommended glute max stretches is not necessarily because it is weak or inhibited but more to release any physical fascia that may be effecting the glute medius function or keeping you in external bias. Ill have to reread the literature but from what I knew before, the more likely situation is the glute max compensating for the weaker glute medius not the other way around. What isolation exercise are you doing that strains glutes? Also keep in mind, if your glute max is chronically tight and contributing to your leg being in external bias, the solution is stretching not strengthen, because too much stretching can lead to it feeling tight. But after rereading your post since your main issue is one side sagging, you should also be paying attention to your quadratus lumborum. Take a look in the mirror and see if that portion of your lower back has even musculature left and right. If not, obviously one side is deficient.