This is awful! I'm so sorry for your losses. I'd be in tears for sure if my budgies started dying like that!
Regarding necropsy, I had an avian vet do a necropsy on my 13 year old budgie, Buddy II, years ago out in LA. He didnt charge me because he wanted to do it because he wanted to examine the organs of such a geriatric budgie who died slowly of kidney failure. Buddy also has a testicular tumor that releases hormone that feminized his appearance for about 5 years before Buddy started looking like a male budgies again. The Dr. suspected that the tumor had outgrown its blood supply and died.
The necropsy showed that his kidneys ere packed with urates they were unable to filter out of his blood. He found the remnants of a shriveled up testicular tumor. Buddy also had cataracts. The Dr learned important things from Buddy's necropsy and I was happy to let him do it. I did not ask for Buddy's remains. I don't have strong attachments to budgies' lifeless bodies plus he had been cut up and I didn't want to see it.
I think the important thing for you to do is to take any budgies and other birds you have to the vet for tests to see if they are infected by anything that may kill them or their flockmates and get treated before they succumb too.
I don't know if a necropsy will tell you anything important. It must be done pretty quickly after the death or the body will have started decomposing and results will be affected. Are their bodies in a refrigerstor? How many hours have they each been refrigerated?
Is the vet willing to do necropies on them and how much will it cost you?