Hi @dave_lynch1
I understand the problem you are facing !
Forcing category URL from the navigation menu is possible - like you need to enable option from the magento 2 backend - Admin -> Stores -> Configuration -> Catalog -> Search Engine Optimization -> Use Categories Path for Product URLs -> Yes
But in this case also first url will always generate in backend and database - as magento generate url rewrite for all the hierarchy like single product url to nested categories URL along with its path as well.
Also i would suggest you to don't change it, as it is for seo purpose.
still for more reference refer this link - https://magento.stackexchange.com/questions/224848/how-to-remove-direct-product-url-in-magento-2?rq=...
Hope it helps!
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