Yeah, that might be a catch-22.. the only way I could get my Z390 to boot reliably was to disable the igpu. Something that might get you past that is using OsxAptioFix2Drv-free2000 but that could also damage your PC longterm I'm told. Maybe someone more knowledgeable then me has a solution for you.
I had the same problem, the update went just fine but after the T6E wasn't detected anymore. Added AirportBrcmFixup.kext and now it's detected and works as before with the added bonus of being recognized as AirPort Extreme :)
The card is a Sapphire RX 580 Nitro+, 11265-01-20G Rev. E7 and my PSU are a Corsair HX750w. The card never gave me any trouble, I just plugged it in (both power connectors) and added the latest Lilu and Whatevergreen, it has been running perfectly ever since.
I bought an RX580 a while ago, it works just fine in Mojave and does what I need it to do. But I also have a HTC Vive for which it really sucks, so I thought to leave my GTX1070 in with the RX but then they share PCIe lanes, both cards set to x8, I guess I'll just have to switch cards physically...