I cracked the final parts of the suspend/resume puzzle for me. I was getting an immediate resume, for which the answer was that my logitech keyboard dongle was causing an immediate wakeup.
- Installing/running solaar made it have the correct behaviour (and still allow me to wake the machine by keypress). That’s documented here on the Solaar tracker (although the answer as to why hasn’t been found yet it seems): How does Solaar affect sleep/suspend and wakeup? · Issue #2893 · pwr-Solaar/Solaar · GitHub
- The nvidia driver has a misinteraction with gnome-shell (mentioned above by @blablablerg , thanks!) which I’ve also documented on this thread: Suspend problem - #27 by fareycircles
- I disabled the lock screen, currently, which I think is preventing the screen from getting blanked immediately during resume.