Thanks; although SDL's documentation wasn't much help, knowing that it was not a ScummVM problem helped narrow down the issue. At first I thought I'd just compiled SDL incorrectly, so I removed it and tried installing through Synaptic. For some reason even though Debian's SDL package includes X11 and Wayland, it still refused to launch regardless of which driver I specified in the environment variables. I searched around some more and found this forum thread
https://ubuntuforums.org/showthread.php?t=944671. The second-to-last post was more-or-less the solution that finally worked. TL;DR for anyone else with this problem: remove scummvm and all versions of libsdl that are currently installed, then install libsdl1.2-dev, download the latest SDL version from
libsdl.org, extract it, compile using
and
then finally
Reinstall ScummVM and it should (hopefully) launch.