@markhughes04
Yes you are right, the onLogin part will be suitable if you had users login on the Wix Stores page, however putting it in the onReady will just create an endless loop, my bad not enough liquid fuel for my brain 
As you are talking about the breadcrumbs on the Wix Stores pages, note that these are technically built into the Wix Stores app itself, so in theory you would probably be best suited going through Wix Support and seeing if they can pass the issue onto the Wix Stores team who can look into this further for you.
You can try having a look at the Wix Location API and the onChange function there.
https://www.wix.com/corvid/reference/wix-location.html#onChange
onChange( )
Adds an event handler that runs when an application page’s URL changes.
Description
The event handler set by the onChange function runs when the location is changed but the change doesn’t trigger navigation. This situation occurs when navigating between subitems on a page that is managed by a full-page application.
For example, a store product page is a full-page application. When a product page’s path changes because it is switching between items, no actual navigation is taking place. You can use the onChange event handler to determine when a new product is displayed and perform any necessary partial updates on the current page.
To determine if a page is managed by a full-page application, use the wix-site currentPage property or getSiteStructure() function to retrieve a StructurePage object that corresponds to the page. If the object contains an applicationId value, then the page is managed by a full-page application.
Also, take note of this page about the back button request and working with breadcrumbs in Wix, with it only working in live sites.
https://support.wix.com/en/article/request-adding-a-back-button-to-your-product-page