I remember xcadey say their spider mount is actually cross compatible with sram 3 bolt gxp mount.
I have 2 xcadey power meters, one was really accurate compared to my home trainer, and the other was completely off. We are talking +100 watts more. However you can adjust the power in their app, so after calibrating to my home trainer it's now perfect.
So in essence, I would only advise to buy an xcadey powermeter, if you have something to compare the watts to.
That price for carbon cranks + pm is really nice tbh. I paid about +400 usd for lexon carbon cranks and xcadey pm.