What’s a good alternative ?

If you're already in the Ableton ecosystem, their newly released stem separation is actually very good, at least for the small amount of testing I've done so far. Much better than demucs, which shouldn't come as a surprise I suppose.

I suppose that depends on the use case.

For mash-ups specifically, using yt-dlp to download music and split into stems with Demucs, using the UVR frontend, before importing into a DAW is effortless. The catch is that you can't expect to get OK-ish separation on anything other than vocals and "other", which really isn't a problem for mash-ups.

https://github.com/Anjok07/ultimatevocalremovergui