Hi friends. Not even a day later and I already solved my issue. For anyone in the future who might need this, here's what I did: After retargeting the Mixamo animation (and confirming it works), export the model as an FBX FROM the ARP export "tab". From there, you can reimport it to Blender, bake the animation in the pose -> bake animation menu, and it should import into RCP perfectly fine from there. Hope this helps someone out because I racked my brain over this for hours.