The movie is mainly the OS. The story is barely a complete story or may not be one at all. Kat has a minimal personal throughline while Protag has even less and the RS is just fumes. I can’t emphasize this enough.
I think the OS/MC problem is Knowledge.
MC: Knowledge of my crime; my husband’s memory of my crime hangs over me. OS: Knowledge of how the tech works is now in the wrong hands.
The OS/MC solution is Thought.
MC: When Kat changes and acts in a way that is Cunning (gist), she is able to change to a do-er, kill her abusive husband, and get over her angst. OS: When Protag is able to think about time / this tech in a new way (when he goes in the reverse direction and figures out how to operate in that direction), then the OS is able to end in success (understanding how this technology works, understanding the caper).
I interpreted it as MC be-er Kat sees IC do-er Protag getting out there and doing something to fix problems and being dedicated to his cause physically in action. IC is known for his past (Brooks Brothers won’t cut it) as an outsider in this secret world.
The RS/IC TL’s are barely there to absent. Whole pieces / signposts appear to be missing. I doubt that it is a complete story.
Edit: This is even less of a complete story than Dunkirk, which was far from a complete story. The focus of Tenet is spectacle and the “going backwards” conceit. It pains me to say it as a fan of Nolan’s previous movies, but it’s his least complete story. Also, rewatch it and notice how it changes from 100% plot/OS to a tiny amount of heart when Kat is first introduced (and every time she has a scene). Kat is the MC, but the OS is the overwhelming focus that nearly excludes everything else.