Since Starts with also matches path components (that’s the abc/xyz/123 part), setting the URIs as you proposed would then result in autofill only presenting the one correct entry for that login page, leading me to believe autofill is behaving well in your scenario.
You might also consider voting for the Vault Item Labels (tags) feature request. Effectively, tags are allowing a vault item to be in multiple folders simultaneously, but with no nesting. In this design, you would likely assign both a “storage company” tag and a “Client 1” tag to your first example. And then, when searching you could presumably filter on both tags.
Until tags arrive, I have (mostly) given up on folders, relying on @grb’s naming trick and searching when viewing/editing because being constrained to a single folder is just too constraining. Do note that @grb is referring to the Item name (required) field and not Username. The item name is free-text and can contain whatever makes it easy for you to identify a vault entry.
