I don't do much audio editing, so I cannot realy comment, But...
I have bought iZotopes RX9 Audio Editor, and that may be worth checking out.
I only use it to separate complete mixes into stems (musical balance), which is working fine. This was my sole reason to buy it.
But you have a lot more tools in there. All of those also as a PlugIn within Studio One. Only musical balance is "stand alone".
Being a Studio One user myself, I share your feeling that nothing else is needed.