I think I'm the only person on Steam with some of the achievements (261/500) because they say 0.0% of other players have them lol

That's a steam bug I think. If you add the rare achievement showcase to your profile it'll probably show up with whatever the real number is.

I think it's called Glacial Storm and it charges up ice as you move and launches them when you stop but on max Agony you basically never want to stop moving so it kind of sucks.

Another bullet heaven I played had something similar (I want to say it was Holocure but don't remember for sure), and the trick there was to just stop for an instant and then keep moving. I'm guessing this one doesn't work that way?

And yeah I've noticed that most games in the genre place a high value on movement with the notable exception of Vampire Survivors itself, where I feel your offense can scale so high that the movement isn't necessary.