Thanks, Karl, for your quick and detailed answers.
Concerning my question on refreshing all previews I didn't express myself clearly. What I meant is an option to have all cache files pre-calculated, so when a folder is visited for the first time the cached files are already there. So thanks for pointing to files.tasks.php, ?task=create_cache&all seems to do exactly what I meant. :-) Did need to temporarily increase max_execution_time in php.ini for this "first run". All future cache updates are of course done automatically, so after this "first run" files.tasks can be disabled again. If I encounter any more caching issues I'll try to investigate why they happen.
Playing with files.gallery I discovered a potential issue. When the default _files directory is placed in the web root folder, there is no protection on directly accessing cached files, e.g.:
https://<url>/_files/cache/images/c5b271.7709838.1660258656.480.jpg
Since the file names are not really guessable this does not really seem a big issue but I moved _files outside of my web root now anyway.
If a webserver has been setup to show directory listings however this becomes more of an issue, then all image caches are directly accessible using https://<url>/_files/cache/images.
Concerning my features request, random slide show of a single folder would indeed be nice. :-)
Anyway, I'll be experimenting further. So far I'm really happy with files.gallery!