This. Lowering you numbers is a risk and doesn't explain why your CUC died.
My first thought with dead snail is dinos. This would also explain the ongoing algae battle. Dropping NO3 and PO4 if those are dinos would make things worse.
1. Urgent- rule out dinos (get a cheap microscope if you dont have on a post a pic of that stuff on you sand bed in the dino thread. They also have a coffee filter method to confirm if it is dinos - but not species specific.
2. Look up the rip clean/sand bed rinse thread. You pull the rocks out, scrub them, spray them with hydrogen peroixde rinse and return. You remove the sand bed rinse until clean and return. But don't do these until you first rule out dinos
3. Whether or not it's dinos, focus on your microfauana, phyto& zoo plankton, bacteria, pods, worms, coraline. these are IMO are part of the CUC. Often, trying to rid a tank of algae can throw these out of balance and make the algae problem worse and even terrible by opening the door for dinos to take over