The final scene of Mark S and Helly R running down the hall panicked and giddy into a freeze frame zoom quotes Truffaut's The 400 Blows. In that film and several that followed, the character of Antoine Doinel, age 14, represents a younger facet of the director himself. He is an innie Truffaut, if you will, coming of age.
In the climax, the rebellious delinquent Doinel escapes from the reform school to which he has been confined and runs to the ocean. He looks around, without any obvious plan for what to do next, before looking towards camera in a freeze frame zoom that encompasses the uncertainty of his future from here.
On a less literal level, I wonder whether the freeze-frame and fade to red connects back to (outie) Mark and Devon's original after-image plan. We get a fading after-image of Mark S and Helly R and the implied question: "Who is alive?" Besides the literal sense when it referenced Gemma, that's obviously a question that goes to the heart of the show. Who is alive?