Site Architecture Issue

Calling all technical SEO experts:

If a site URL is in the following format:

mysite.com/category/topic/post
and the /topic/ is actually a 404.

So basically mysite.com/category/topic leads to a 404
but mysite.com/category/topic/post is just fine (Leads to the right post).

What would be the best way to resolve this?

To populate /topic/ with an actual page with content related to the topic and deeper links within the site (Like a sitemap) or just leave it be . . .

My theory is if I add a page, it will close the 'loop' for Google so link juice will flow more appropriately. Plus with deep links on the page, it will only help other pages rise as well.

I would love to just remove the /topic/ thing completely and do a 301 and enjoy the smaller URL structure but we're kind of been using the same architecture for almost 2 years.

Ranking have been increasing but super slowly.

I think if we get rid of it or replace it with a page (Little content and deep internal links), it might do the trick without us losing traffic vs if I just deleted that /topic/ slug completely.

What do you guys think?

Better to take a hit for a few months and have shorter URLs without 404s

OR

Populate the 404 page with some unique content and use deeper internal links on the page.

PS: This is one JUICY TECHNICAL SEO question for sure

Do you have any links to mysite.com/category/topic on your site (breadcrumbs etc )?
I'd rather make 301 redirect.

Thanks for your response Denchik. Yeah that's the thing . . . we have tons of links to pages under mysite.com/category/topic - 90% of our site. It's moving up but super slowly. I think it's this Site Architecture that's slowing the growth. So the easiest solution it seems like is to simply popular these 404 pages with unique content . . . What do you think?