Well this is embarrassing. I found the fix for this when revisiting it today and apparently googling just the right part of the error, which found a few similar issues that people had had… including a post that I myself made less than a year ago, to this forum, for the exact same thing:
(Getting older sucks. I noticed getting a little more forgetful… but getting stupider is a surprise, because apparently I figured this out on the same day last time.)
What happened, then as now, is that an update between making virtual servers added PHP 8.2 and I didn’t notice. Since my script defaults to “use the newest php”, and gd must be installed manually (apparently), I fixed this the same way I did last time (though adding .1):
sudo apt-get install php8.2-gd
You’re welcome future-Ron.