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.

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

Have you looked into Pseudotv / PseudoTV_Live?