I’m not sure what material or style these steps are, but they’ve started crumbling after water got into a crack and froze during the winter. It seems to be getting worse and I’m wondering how to address it. Is this something that can be patched or do I need to remove all this material and start over?
Yeah looks like a "remove it all and redo it".
I'm not a specialist though so a pro from your region is probably what you should listen to.