The old karaoke machines youd buy at walmart back in the day traditionally used CD-G's to store the low res karaoke video.

Look for CD-G iso/zip torrents on your favorite tracker. There's a massive pile of rips out there, I know I myself downloaded some 1000 song collection once. VLC can then play CD-G formats.

Likely not the answer youre looking for but pi karaoke is pretty awesome for a setup. It’s a simple install and scrapes youtube for videos, but since it stores them locally you eventually have a decent library of the stuff you like.

https://github.com/vicwomg/pikaraoke

UltraStar Deluxe

Rigmar has a pretty large collection (around 500GB when I downloaded a couple years ago) of most of the famous songs you’ve ever heard. If you have the bandwidth and disk space I’d go for that. I use cdgtools to burn to CD-Rs or just open them in VLC.

I just find instrumental only versions of songs I like and just sing over them since I don't have a karaoke machine and I don't see a need to get any fancier than that. Do you really need the bouncing ball/filling out text pacing you for a song you already know? Does anyone sing karaoke for songs they don't actually know?