Thanks, that made some progress. Without AMDRadeonX4000.kext, 10.14.5 will boot, it recognizes the card as an RX570, but there's no acceleration (as you said). This is the same with or without Lilu/WhateverGreen. I noticed that the X4000 GL driver contains pcmpgtq instructions, so I'm guessing that's why removing the base X4000 driver allows it to work at all.
re: crash logs - with DisableLibraryValidation, Lilu, and WhateverGreen loaded, I don't get any crash logs - it just hangs forever at the first progress bar (white background). Nothing to analyze, just a quiet failure to boot.
Since I can now reliably boot with the RX570 without acceleration, I'll move on to just testing my stuff. Thanks for everyone's help (although if
@Ludacrisvp was to pass along his Lilu/WhateverGreen settings, I'd surely try them out).