It’s not web based but MKVToolNix GUI is pretty user friendly. I haven’t dug into it too deeply so I don’t know if it offers any automation tools to batch change files etc.

In case you haven’t already seen it yet there’s The Password Game to drive this point home

I whipped up a basic page with PHP and just used XAMPP when I was on Windows. I recently switched my daily driver pc to linux and haven’t updated it yet. I only used it to save MP3s, videos at yt-dlp default “best available” settings, and a custom option that lists available video/audio formats where I can specify the ID to grab of each. No validation or sanity checking etc, just some switch statements and basic form functions.

main page

custom selection