Thanks for the replies,
as Fullretard said, overlaps is the issue that kills me if I wanted a 100% accurate calculation or simulation.
The thing is, I know how to "exploit" the mechanics of PPM and PPH to increase the uptime, but I can't calculate the actual uptime of the buffs to simulate my DPS in order to compare items.
I'm afraid it has to go in this direction: https://iam.yellingontheinternet.com/2012/10/16/theorycraft-101-how-to-compute-uptime-of-a-proc-based-buff/
But since I'm a pleb in maths someone has to break it down for me ;)