Same nightmare here. Hangs, mouse moves, but everything else is frozen.
Almost identical system to yours. The funny thing is, it ONLY hangs when using 2 finance programs, which are both heavy JAVA/GPU. No overheating either. I run LuxMark stress for hours, no issues, tons of other programs basically killing the machine with Unigine, etc, all solid without those financial programs, runs all day with no issues. Windows 10, same apps, all day no issues. On Mojave, once I put back the RX580, runs all day. So it's strictly with Radeon VII and the latest 10.14.5 Mojave and those finance programs. I noticed a pattern where Safari was open in most of those cases. Very odd.
Going to try those settings above. Worse case, I might just go back to High Sierra and the 1080Ti.