I have a Lenovo A485. Kernel 4.18.12 works fine for me, but I have tried almost
all the RC's (1, 5, 6, 7) and the screen turns black when GRUB would normally
be showing "loading initramfs" or it freezes when it's showing "loading
initramfs".

lspci -nn
05:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc.
[AMD/ATI] Raven Ridge [Radeon Vega Series / Radeon Vega Mobile Series]
[1002:15dd] (rev d0)

I found this bug report which seemed similar to what I was experiencing, but my
firmware is up to date https://bugs.freedesktop.org/show_bug.cgi?id=107880
[    2.814558] [drm] Found VCN firmware Version: 1.73 Family ID: 18

I tried:  modprobe.blacklist=amdgpu and that did not change anything.

Let me know how I can help troubleshoot this problem. Thanks.