I'm not really sure why you would want to use modular arithmetic to describe DoT truncation. Unless you just encountered it in an introductory number theory course and were really excited about the concept or something.

You probably could design a DoT focused job from the ground up that conforms to the 'rules' of job synergy. Time magic would be a great way to do it. Perhaps you freeze your DoTs so that they continue to tick without the timer changing, and then accelerate all the timers to do burst or deal with phase changes. I don't think that clipping is that much of a design problem with DoTs either, as you can simply design timers that can be extended twice in a row. It would be interesting if the focus wasn't on actually recasting DoTs, but rather utilizing a set of buffs to build up the momentum going into burst.