Hi all, I’m curious to know which one of the following cases could improve PPA more than the others in custom design: 1. Using lower layers of metal for routing => complicated routing 2. Using upper layers of metal for routing => complicated routing 3. Using a combination of 1 and 2 for routing => simple routing 4. Other cases?

Its hard to say. But in normal designs "simple" routing is gonna produce plenty of (unfixable) DRCs. You may need to decrease the density of design, therefore worse area utilization. Using lower layer of metals may decrease the PPA because it has worse resistance. Using higher layers may improve PPA, but since higher levels are simply less dense, chances are, you will get routing congestions and DRCs, therefore worse area utilization. There is no good answer here imho