Unfortunately CMS URL’s in Webflow are strictly structured as; /collection-slug/item-slug
There are no other options built in to Webflow.
When businesses really need differently-structured URLs, they will typically setup a reverse proxy solution. If you search the forums you’ll find some discussions of that, it’s fairly technical but will give you a lot more URL flexibility.
If it’s just a personal, fun site, you could use a script based approach.
However in your case, I’m not sure how any of those would work, because [user_name] won’t be especially easy to do lookups and redirection programming with.
I’m curious is the [user_name] you’re referring to a slug in a ref-linked Collection?