What use case does a million character PR have?
The "surprise, you can't review all the files in your PR" using GitLabs standard web based tooling makes it a no-go.
With GitLab, when you hit the rate limit, any file "past" that limit doesn't even show that it exists in the MR. It just looks like the MR is missing a bunch of stuff, with no workaround available. :( :( :(