Both GPU's "working" does not mean dGPU is encoding...it will decode but not encode WITHOUT the dummy kext (unless you are using 10.14.5 BETA).
You can prove it to yourself by examining the IOREG WITH and WITHOUT the dummy kext.
For your CPU, it's better to use iMac13,2 - it's the closest match.
For proper CPU power management, you need to set the right config parameters (XCPM or SSDT.aml) but that's outside the scope of this thread and you should start a separate thread in the desktop section where others will help...
Your options are:
1. Enable IGPU in BIOS, Inject->Intel=Yes, and use connector-less ig-platform-id=0x01620007. Or,
2. Change SMBIOS model type to iMacPro1,1. But you will get green glitches in the H.264 output. Or,
3. Keep your SMBIOS as-is and add boot flags: shikigva=32 and...
NOT correct advice.
WITHOUT the dummy kext, the RX4XX/RX5XX will not be utilised and you will only get Intel Quicksync/Software encode/decode which is fine and will work. BUT, that's the not the purpose of this thread here (which is to try and use the AMD GPU for encode/decode).
I am on MacOS 10.14.4.
I enabled the IGPU connector-less (needed enable H.264 encode/decode WITHOUT green artefacts)
I installed Dummy141.kext in EFI/Others (needed to enable HEVC encode/decode in RX580)
I use SMBIOS iMac13,2
I have DSDT.aml and SSDT.aml (for CPU power management)
OK the first screenshot with the Intel Power Gadget, shows that you have IGPU enabled (the green graph line).
It also shows that the CPU is maxing out at 4.0GHZ.
This implies that even though everything "works" and "perfect" for you, your system is using the CPU and IGPU to do the encoding (i.e...
Those screenshots show that Premiere will do the h.264 encode but it does not show with which GPU.
You need to run tools to show the loading the IGPU and the dGPU. (e.g. Intel Power gadget and Activity Monitor).
If H.264 encode is truly being executed by the dGPU then, it's load & utilisation...
It's not sufficient to say "perfect" without first checking what is actually doing the encoding.
If you have IGPU enabled, then you need to confirm if it is doing the encoding or the dGPU.
That is, you need to prove to yourself that the encoding is done either by IGPU, dGPU or both.
You are having lot of success BUT you are still have IGPU enabled with iMacPro1,1.
Check the IGPU & RX580 loading while encoding H.264 to HEVC, H.264 to H.264 and HEVC to H.264.
That is, is it really using the RX580 or IGPU or BOTH ???