I would use one of the red or blue USB slots, not the 4 at the top that are USB 2.0.
1. Ensure your USB installer is actually working and was made properly, without errors. Most install problems are because someone made a USB installer through some sorted hack and slash method. Make one on a real Mac that has been updated to the latest OS you plan to install. Anything else may inject errors that will either fubar your install or prevent you from even installing.
2. Make sure your BIOS (not SMBIOS yet) is updated to the latest version (F12d for the Pro and F12c for the Pro WiFi). After updating the BIOS, go back into it and enable the dGPU, save the settings and reboot. Go back into the BIOS and then set the parameters of the video card, which will now be visible. Save your settings and then load from the USB installer.
3. Download either the Clover or OC EFI that is appropriate for you (ones for the navi card). I suggest Clover as it's far more mature than OC and has more tools that support it.
4. Open up the config file and fill in the following (for Clover): 1) BoardSerialNumber, 2) SerialNumber, 3) SmUUID and 4) ROM. Then add "-v" to the boot arguments in case you have any errors, you can see where the process halted. Once your system runs fine, you can remove it get the nice Apple graphical boot process.
Many of us have identical systems. Yours sounds very clean and simple so there shouldn't be any problems. I would go back and make sure you're doing all the steps properly.