How do you open multiple videos with multiple subtitles in VLC from command-line?
This is a workaround which converts the files to another container format. It happens almost as fast as your hard disk can write data because nothing is re-encoded.

use MKVToolNix (specifically mkvmerge) to combine the mp4 files and their srt files into mkv files (perhaps you can automate this with a loop). I never use this tool via command line and cannot tell you the exact command, but you'll find more information here https://mkvtoolnix.download/doc/mkvmerge.html
use vlc.exe *.mkv afterwards.

Maybe this question helps too https://superuser.com/questions/609113/how-to-add-and-remove-subtitles-in-an-mkv-file which should also work with an mp4 file as input.