It's not so much a matter of calibration but a matter of accuracy. I agree with @jmudrick that external microphones pretty much always will be more accurate.so has it been determined yet that it's even worth purchasing and using a calibration mic like the imm6, or is it just as effective to use your phone with the case off.. eg. Samsung s23?
The term "calibration" is used a little ambiguously in this discussion. There are different aspects to it that are somewhat related: First, calibration can ensure SPL readings are correct in absolute numbers. We don't really need that for room correction, but it doesn't hurt either. Second, calibration can correct for frequency dependant inaccuracies of the microphone, ideally for each individual sample. Having that would be a good thing, but the better the mic, the smaller the error, that would need correction.
Personally I would always go with a USB mic. Not only don't you need an external ADC dongle. You also get the correct absolute SPL "for free" without the need for a calibration file. Even if this is not required for RC it allows more flexible use.