Updates: so it’s been about a month, and the blue algea hasn’t come back since I treated it!
I dosed all my tanks that were showing signs and not a single one has an outbreak anymore~
Only one fish has died since: my albino pleco. But I’m not sure why, she suddenly stopped eating and I tried putting a variety of chopped veggies in and nothing. She just slowly got paler and paler over the course of a couple weeks. I’m really not sure what happened though. All of my other bottom dwellers/shrimp are okay so even though it’s not impossible, I think it’s unlikely that she managed to eat an undissolved particle.
I don't know where the sand issue for cyanobacteria came from, it pops up here and there, but there is no evidence it causes cyano. I had soft play sand in my 8 tanks for a decade and the only tank that ever had cyano had it near the surface on plant leaves.