At the very least, Mary was the most blessed woman who ever lived. She got to be the mother of Jesus, the Son of God. That we can all agree on.
The Immaculate Conception is a problem however. The Orthodox (who also don't have the doctrine of Original Sin) view the IC (for short, no disrespect) as another western innovation. The Early Church Father's don't really talk about her that much.
And some of the justifications for the IC are a bit gnostic IMO. Saying that Jesus could not have been surrounded by a sinful mother implies that He could also not have been really present on the earth. For whatever Mary was, the pagan Roman world made Vegas look like a Sunday School picnic.
There are much bigger things to debate, but this is something that is a real barrier between for instance the Eastern Orthodox and the Catholics. Not to mention the Lutherans in most synods.
The perpetual virginity of Mary is a silly thing to fight over though. Most Lutheran synods have taught that she did not have any more kids, and that Jesus’s brothers were more likely half brothers from an earlier marriage of Joseph's. The angel told Joseph who his adoptive son really was, and who was the Father. It isn't that big of a stretch to see that he would not want to lay with her after that!
Not sure what Lutheran branch you are speaking of, but I can say emphatically that the Wisconsin Synod Evang. Lutheran (WELS), definitely does not teach that Mary had no other children or that other siblings were from a previous marriage of Joseph.
They lean towards Jesus having half brothers and sisters conceived by Mary & Joseph. WELS stops short of claiming this as complete truth only because the Bible does not outright state this.
God Bless
" But Orthodoxy does not admit in the all-pure Virgin any individual sin, for that would be unworthy of the dignity of the Mother of God." Sergius Bulgakov, The Orthodox Churchyet they also reject the idea of Original Sin, so the net effect, to my understanding is "Mary did not have any individual sin and there is no Original Sin so she must have been sinless"