Does a keyboard shortcut exist for creating sub-collections?
Per https://github.com/zotero/zotero/issues/2899#issue-1439595808, I presume not. However, the ZForums appear to be the sole way to officially request features (because GitHub usage is restricted).
Upgrade Storage
If you mean if there's an individual keyboard shortcut, the answer is no, but you could likely configure the Actions & Tags plugin to get you one.