iMac18,3 and IGPU enabled - h.264 hardware support, yes. (quicksync) HEVC hardware support, (quicksync). iTunes DRM, no. (i want the rx580 to encode HEVC/i want iTunes DRM)
iMac19,1 and no IGPU - h.264 hardware support, no. HEVC hardware support, no. iTunes DRM, no. all done by software.
iMac19,1 and IGPU enabled - h.264 hardware support quicksync. HEVC hardware support, yes (quicksync), iTunes DRM, no.
iMacPro1,1 and no IGPU - h.264 hardware support, no. HEVC hardware support, no. iTunes DRM, yes. (correct).