This sounds like our kidding season with our goats for the past two years. The first year I suspected the buck also - it was the only change. I thought it was just poor genetics, but I missed the signs in the does. It took 5 years for the selenium deficiency to rear its ugly head. We sold 2 does before they kidded last year - they were the oldest of our herd, so probably the most deficient. They both had quads (which didn’t help), but the buyer lost 5 of the 8. She almost lost one of the does.
Hearing that the ones with more lambs seem to have suffered the most, and that they were weak in the hind end also sounds like selenium. We also had an early abortion, and hard sacks around the kids - which caused some deaths - as well as still births. Any obvious problems with contractions? Our does had weak contractions that caused some of the still births - they just couldn’t get them out fast enough! We ended up doing a lot of pulling.

This is also caused by selenium deficiency. I didn’t know anything about selenium, so I missed all the first signs.
After some great advice, we gave selenium to the does still needing to kid and our results improved. Kids were still weak, but bounced right back with B complex and more selenium.
I hope the answer is as easy for you! What a traumatic season!! I’m so sorry for your loss, and hope the support here can make a difference.
check your selenium! Set a regimen to give selenium supplements with your loose minerals if needed. We have had to do both. I know what we use is not recommended for sheep, so I won’t go there. Hugs!!