How can I diagnose my broken X server (Parabola/Arch)?

I am using Parabola GNU/Linux (based on Arch) on an x86 desktop with an nVidia GTX 1050 GPU. I ran pacman -Su yesterday to perform a full system upgrade and it seems to have completely broken my X server. Now, on boot, the display manager (sddm) won't load and it drops into the CLI. I have searched through all the log files I can find (dmesg, Xorg.0.log, rc.log etc.) and I can't find any clues as to what the problem is. I should also mention that I am using OpenRC, rather than systemd. Interestingly, if I try to load the DE from the terminal with startx, I get different issues with Mate and LXDE. With Mate, I get a black screen with a mouse cursor that responds to the mouse. With LXDE, the desktop, panel and background all load up, but I can't see the mouse cursor (although right clicking brings up the context menu). Any ideas what I can try to diagnose the issue and fix this? **Update:** I found the ~/.cache/lxsession/LXDE/run.log file, which indicated that lxpanel was having an issue with libxml2. I downgraded that library and now LXDE seems to work ok when started with startx. However, it doesn't seem to have fixed the issue with Mate. I have now realized that Mate is actually working, but is experiencing a different issue. When I log in with startx, I initially get the black screen w. mouse cursor; however, if I switch to a different virtual terminal (e.g. with CTL-ALT-F2) and then switch back again, the whole Mate desktop is there and working. I believe Mate session errors should be getting posted in ~/.xsession-errors; however, that file isn't being touched. Does anyone know what might be causing this issue? XDM/SDDM seem to still be crashing on boot as well, so I need to figure out what is going wrong there.