I think replacing the spider with a power meter is your best bet. I have a Power2max
on two cranks and a Quarq on my Sram Force crank. They have all been great.
I wouldn't recommend the Stages because it's notoriously an underperformer, not as accurate as a dual leg, and you will have to spend the extra money on a new spindle as well which makes it not a cost effective solution.
The SRM is how you spend twice the money for something that isn't any better than a Power2max or Quarq.
I was just looking at the power2max site, they offer a SRAM power meter spider but it say not compatible with sram 1x cranksets......Do you run 1x or 2x with the power 2 max meters?