Multisite subdirectory, possibly htaccess question

Was debating between Multisite or not to avoid this hurdle, but I believe this is wisest. Agency needs their main site then each staff member’s portfolio, which is a totally different theme, different nav and they may want to limit their access. So I feel Multisite is the simplest solution. (If you don’t let me know a better one, thanks)

I considered a theme switching plugin, but those don’t work when you’re attempting to use a theme’s portfolio custom post type thing. Since we have galleries (different) for both staff and agencies I feel a single site is ruled out at this point. Especially anticipating user access control..

Ok, what I’m attempting and can’t find an answer to.

I need a URL structure like this : Site.com/staff/first_name (first name would be the site name)
I’ve been banging my head against the wall trying to figure out how to add /staff/ to the URL in a graceful manner. I’d obviously be using subdirectory, and not subdomain.

What I’m hoping to find:
A line of code to add to wp-config when enabling Multisite -or-
An htaccess solution -or-
Something I haven’t thought of that you may know

Also, would save me from merging two templates if I can make an exception for site.com/blog so it is a subsite as well.

I feel like this is super simple but have been failing to solve it myself. Thanks for your input!