For dj use I recommend Nuo Stems, as it integrates with dj software. But you don’t need it if you only want to play with the models behind it. That’s because it uses mel_band_roformer and bs_roformer which are really great stem separation models (i haven’t heard anything better yet). Their page about stem separation quality is a nice place to start if you want to dive into these types of models: https://docs.nuo-stem…
Doesn't appear to be free or open source unfortunately.
Compared to the prices and licensing hell of NI and other dj brands, just paying 35 euro for a perpetual license to solve a problem and always have the best models delivered to you is actually nice. The price is fine for what it does and the consistency, support and positive energy that comes with it.