The small snails could be baby snails or they could be snail eating pyramids. Are you finding the tiny snails on the dying larger snails?
I am having a die off as well. Two seperate shipments from an online vendor and some from local LFS are just dieing!
I have had pyramids who prefered snails to clams, but they are not at infestation population levels any more. I waited along time to add new snails.
If I add snails, within 2-3 weeks 80% are gone, "hermit snacks", they just drop off the glass and rock. All water params are good. How are yours?