TRP Spyre SLC brakes compatible with other vendors' rotors? Does anyone know whether TRP Spyre (SLC) mechanical disc brakes can be used with other vendors' 160mm rotors, i.e. the ones for SRAM's HRD brakes?
Yes, they are compatible. As long as you are using a 160mm caliper mount, and a 160m rotor they will work together. There are industry-wide standards for rotor position for front and rear brakes.

Like @Carel says above though: make sure your pads are a type that are correct for the rotor (this shouldn't be a problem with HRD rotors, you can use metallic or organic pads on them) and also compatible for the type of riding you are doing. For example, if you are doing tons of downhill with organic pads, you'll wear through them really quickly.