Well if you do change the URL and setup the 301 redirect, you can always change, and should change, all the internal links to point to the new URL.
As far as losing any PR, Google announced back in 2016 that 301 and 302 redirects no longer lose PageRank. You can read about it in Cyrus Shepard's post, https://moz.com/blog/301-redirection-rules-for-seo.
He states:
"While it’s super awesome that Google is no longer “penalizing” 301 redirects through loss of PageRank, keep in mind that PageRank is only one signal out of hundreds that Google uses to rank pages.
Ideally, if you 301 redirect a page to an exact copy of that page, and the only thing that changes is the URL, then in theory you may expect no traffic loss with these new guidelines."
It sounds like the only thing that will change in your situation is your URL so you should be able to move forward with confidence that 301 redirecting /marvel/avengers/hulk to /marvel/hulk won't have any sort of negative effects, (at least not long lasting).
But if you're still feeling cautious, it's fine to not change this one URL.