That is frightening. I only have my first GSD and she's still a puppy (so obviously I'm way ahead of myself, lol), but based on all the GSDs I've known and how different this puppy is compared to others I've had, I can seriously see myself breeding them in the (far!) future. It really sounds like it would be full of heartbreak, though. I'm so sorry for the health problems in your litters. I can see why that would make you take a step back even from a breed you love dearly.
The big issue is epilepsy though, which is thought to affect up to 20% of the population, also a risk in all varieties. Of course, statistically that is about the same as GSDs affected by HD but statistics don't really work that way. <knock on wood> I have never had a Belgian who seizured (had a GSD who did though and bad) and in 4 litters only had one puppy who did (litter sizes were 6 puppies, 8, 5 and 2). I've known litters in which 3-4+ puppies seizured. So in real life it doesn't work out to be 1 in 5 at all, like with HD a lot of other factors matter.
Work is being done to find a marker for epilepsy in the Belgians: http://www.sciencedaily.com/releases/2012/03/120323205337.htm
I had three GSDs, two American lines and one German showline. One of the American line dogs was put to sleep due to uncontrollable seizuring when he was just under 3 years old. Heartbreaking for sure The other two lived long, healthy lives passing away at 13 and 14 years old. Of them, one did develop moderate arthritis in old age. None of their siblings lived as long as they did - many reached the 10-12 year mark though. I never xrayed either of them but they didn't seem to have HD or ED. The two Amline ones had issues eating poor quality food (corn based, store brand) but once switched to raw did much better. The Amline girl actually had multiple hot spots and ear infections already by time I got her at 15 months and once switched to raw food, never had them again. I didn't even have to be at all careful with what I gave her as treats past the first year - the problems just disappeared. She had to be switched to kibble the last 6 or 7 months or so before she passed away and she did fine on it. The other one never had an ear infection and only had a couple very small hot spots after we were eaten alive by biting bugs while camping in the Adirondaks. Her relatives had some health issues though. In a litter of 10 one seizured, one had HD and one had DM. Her mother developed EPI a year or so after having the puppies. AFAIK none of the puppies developed it.