I googled "anti-fya" and found some basic information about it. Basically, it is similar to a Rhesus antibody sensitivity because it can cause hemolytic disease of the newborn (HDN). This causes the growing baby's red blood cells to die and can build up bilirubin in their blood which they are unable to excrete from their environment (it ends up in the amniotic fluid) and it can also cause the spleen, liver and heart to become enlarged. Although it is directed at Rh factor HDN, this page
has some very good information about HDN. It doesn't give any percentage risk or percentage associations of anti-fya antibody presence and actual cases of HDN, and you'll probably need to do more research to determine if there is any research in this area. Even this kind of data is questionable and you will probably come up with all ranges of numbers.
Although amniocentesis is mentioned as a screening tool to help watch for the development of HDN in a growing baby, I share your hesitancy in using it. It does carry a higher risk of danger to the baby, and I would not consent to it lightly. Higher level ultrasound seems to be quite effective in monitoring for organ enlargement that is associated with HDN, and since it is considerably less invasive than amniocentesis I would probably agree to more frequent scans. I would highly recommend you find a perinatalogist who is willing to work with you and your chosen care provider to develop a flexible care plan and provide you with all appropriate research so you can make informed choices about your and your baby's care.
sending you peaceful vibes...