"Your system's hardware does not support hardware acceleration for the current settings" on export

The MPE renderer and the Intel H.264 encoder are two completely separate things. In your case, the Radeon R9 M295x, AFAIK, does not support hardware H.264 encoding at all. And the H.264 encoder in Premiere Pro CC 2018.1 only supports 6th- or later generation Intel I series CPUs with only the integrated, on-CPU Intel HD or UHD Graphics 500- or later series enabled. Unfortunately, Apple configured all of its systems with discrete GPUs to always automatically switch the GPU to the discrete one when Premiere is launched, and makes it very difficult, if not impossible, to override that behavior. And Adobe does not currently support hardware acceleration of H.264 encodes through a discrete GPU.

By the way, your iMac is too old to even support Adobe's implementation of the Intel H.264 hardware encoder in any event: It has only a 4th-generation i5 or i7 CPU. And even its integrated graphics is only an HD Graphics 4600 – too old to be supported.