I don’t want to take away from OPs project (which seems really nice) but have you tried Ultrastar Deluxe ( https://usdx.eu/ ) in combination with USDB ( https://usdb.animux.de/ )?

There’s also a program for automatically downloading the songs: https://github.com/bohning/usdb_syncer

What would you consider the best "Singstar-like" alternative atm?

please try it out and share your feedback afterwards!

both transcript/alignment might not work perfectly, but it really depends on the song