Flatten URLs (remove subdirectory)

(How) is it possible to remove references to collection subdirectories? For optimal SEO, we want to have each landing page be linked from the root (site.com/landing-page123.html) versus (site.com/collection/level2/landing-page123.html).

About 720 pages in total.

Can we achieve this with a 301 redirect?

720 of them, yes.
But you don’t need to.