The main thing to note is that Dusan's Serbia was crushed in battle before it even gained it's momentum.
Historically Dusan's father did a lot of damage to the Romans, and while this was temporarily reversed, the Serbs were able to steal all that land again when the Romans went into their civil war in 1341.
Within the TL Andronikos and his men crushed Stefan Decanski before he, or his heir Dusan, were able to do much damage. Dusan's effectively inherited a Kingdom that lost it's last major war and is still rebuilding its army.
The fact that Dusan won't be able to rely on the 1341-1347 Civil War as a springboard to yoinking Macedon, Thessaly and Albania means he's never going to be as great of a threat to the Romans as he was OTL.
Also, glad you're enjoying the TL, thanks for commenting x3