I did see somewhere that the 60hz limitation was because of NTSC and powerline frequency (hence why I made that bad multimeter joke earlier). I personally like higher frequencies, but it's not my main concern.
I'll decide exactly what I'm going to do, but I'm going to avoid frame-based programming, to minimize these complications. However, I will continue with tests at 60fps to be closer to the experience most players have.
I'm going to have to come back to the topic of performance at some point, to avoid what happened in your experiment happening, the box method is something I'll be resorting to for sure.
Thanks again for this great conversation, it was a good learning experience! If I manage to finish this project as I want, your name will be in the thanks in the credits and I will give you a game key :)
If at some point you are launching a commercial game, talk to me too, depending on the situation, I can help a little with the promotion.