Keep in mind I only really care about the relic. I'm rank 13 now and am at the relic stage where I'll have to run DR 15 times (ugh), and I assume that's gonna cost a lot of mettle.

So, should I grind out rank 15 before I sink all of my mettle into this, or does it not really matter?

I mainly worry about them adding ranks or mettle requirements to whatever is left of the relic chain, and I'm not really sure how likely that is to happen.