What control node specifically and how should I use?
Specifically for what? Well, "I want something like Ableton" is not a concrete question and requires too much efforts (including guessing) to answer.
Aside of this, from what I understood, you could use just
Control and draw on it with
CanvasItem methods or
ColorRect + same + shaders. UI in Godot is very powerful and complex (and sometimes quirky too). You can use themes and individual styles, and sometimes combinations with Node2Ds too :-) to achieve required look. The power of
Control and its distinction from
Node2D is that they designed to wok together in layouts.
Again, I think it's more practical, if you'd begin learning (by starting a prototype project) and once you hit a real problem, you'll be able to form a specific question(s) for a collective benefit.