More fiddling with BookStack. Backing up to Wasabi is working fine and has encouraged me to clean up my backups a little.
Export to PDF just isn't working, even when trying to get it to use wkhtmltopdf instead of the in-built PHP PDF renderer. Nothing in Apache's error logs, just a jaunty Laravel fake HTTP 500 message on the page (error mirrored in the dev console).
Not critical but, sigh, one more thing to add to my list of things to investigate and fix.