There are many power management bugs open.

After a ton of testing, I've determined that the weak link in the chain is gdm, GNOME's display manager. I have found a working-quite-well configuration with the latest elogind + sddm. This allows my Thinkpad P1 to successfully suspend/resume which is promising and an improvement on prior behavior.

Based on this research, it appears best to move to sddm as our default display manager. It works well with GNOME and Cinnamon, and obviously KDE.