convergence-components VS mixxx

Sailfish applications use Qt's QML declarative language for their GUI. Sailfish has a proprietary QML library in addition to the QML elements included in Qt (Qt Quick). What we need to do is decouple Whisperfish's QML from the Sailfish QML library (which is called Silica). Fortunately PureMaps has already done the same thing and made a QML library that abstracts over the QML libraries of Sailfish, KDE (Kirigami), and Ubuntu Touch.

There’s also this project to bring SailfishOS apps to other mobile Linux platforms https://gitlab.com/dylanvanassche/convergence-components that might be applicable here.

Mixxx is excellent and has pretty wide hardware support.

FYI the "limited" functionality of Rekordbox can be hardware unlocked with some Pioneer controllers.

https://mixxx.org/

There are quite a few DJs that use Ableton for DJing (or hybrid sets). Of course, Ableton wasn't made with DJing as its main function in mind but you totally can use it for that purpose and (with some tweaking) do everything and more that would be possible in a dedicated DJing software (like Traktor or Rekordbox or Mixxx)

I’ve used Mixxx a little bit and have been surprised how capable it is. It’s an open source project and is free to download: https://mixxx.org/

Mixxx (this one's free)

Mixxx

Related works: https://github.com/mixxxdj/mixxx/pull/4027

Mixxx (you already mentioned this; it's certainly a great piece of free software to test the DJ waters with and works with a lot of gear).

Alternatively, you can also use Mixxx with the S4 MK2. Mixxx is free. But instead of a Traktor control vinyl record, you'd use Serato timecode instead, as stated in the docs.

If the device is class-compliant, i.e. it follows the generic USB spec and doesn’t need drivers installed, it can work under Linux. However the free software you get with the controller (Serato, Rekordbox, Traktor etc.) won’t. The alternative is to use Mixxx. It’s free and open source. You can check their controller support. https://mixxx.org/

If you don't like any of those, aren't you glad you didn't spend the money to find out the hard way? There's more DJ software than those two, so try out these as well: Traktor Pro 3, Virtual DJ, DJUCED, or Mixxx.