No matter what you do to the filenames the metadata of the images of when they were created will be used by iMovie when you import them so they always get sorted by date, old to new in iMovie.
I got round this first by copying all (1,021 files for me) of the files to a test folder to work on in case it went wrong. Next randomise the filenames, I did this using a MacOS terminal and the below first "for loop", then strip all the date info from the metadata using exiftool (download it).
for img in *.jpeg; do newname=$(head /dev/urandom | tr -dc a-z0-9 | head -c 8); mv "$img" "$newname".jpeg; done
Repeat for .jpg .JPG files etc
exiftool -overwrite_original -time:all="" -wm w *
"*" in the last does all files in the dir. Then import from the dir into iMovie. Quick and dirty but works to give a shuffled order in iMovie.