Okay, an issue here is that The Empire operated much differently from the Republic in terms of military campaigns.
Unlike the Republic, which actually relied on planetary invasion for success, the Empire used orbital bombardment to obliterate planets and systems. There was not such a thing as planetary invasion and surface forces were only used to aprehend important Rebels, secure bases and minor stuff.
Given the fact that the OP has ruled orbital bombardment out, then it would be logical for the Empire to change tactics and actually plan a military invasion in where prior reconnaissance was performed.
If the Empire sends 10K storm troopers (which it perfectly can), maybe some hundred AT-STs and a couple squadrons of T-fighters, Vader can decimate LOTR planet.