Its been a month since the OP, but I had some thoughts I'd figured I'd share anyway.
At the absolute highest level of this industry mixing, technique has very little to do with audio processing tricks. When I say absolute highest level, I'm talking about media projects that enter the audio production process with $100,000,000 to $180,000,000 budget and pull gross revenue sales in the billions off direct sales, not including licensing royalties and streaming revenue. $1 billion + on direct consumer sales.
A studio or mix engineers ability to handle projects at this level is defined by completely the scale of their facility and their experience managing workflow. The design, recording, implementation, and mixing of the sound is all secondary to the methods by which the product is completed. Parts of the performance, design, and tracking may be extremely complex, but the mixing itself... its pretty basic. Automation. A little EQ. Reverbs. Minor Compression. Panning. That's all. At the audio industries highest tier of mixing, the pluging choices and techniques are hilariously simple. All of the heavy lifting is done AND DONE CORRECTLY before it gets to the final mix stage.
High level mixing facilities have tools that you will never see in home studios. And they're quite necessary, but only in the sense that they allow enormous teams of audio professionals to collaborate. They're work flow tools - and have nothing to do with sound processing. The arguable exceptions is the monitoring.
Its an oversimplification to say that only the person matters. The worlds highest paid mixers can not mix in a bedroom with a laptop. The suggestion anyone can is a widespread blatant fallacy that only comes from people who do not understand what goes on in a multi-million dollar production room. Self proclaimed 'audio experts' thrive on the ignorance of the general population, happily selling this pipe dream to DIY-ers. You can make an outstanding mix in a bedroom, but don't expect it to compete with anything that a major production company has sunk eight or nine figures into developing.
Regarding advanced techniques
- The majority of highly complex and creative techniques are actually implemented in what most people here would call the tracking stage. There isn't a huge distinction between the tracking phase and sound design phase, though the sound design phase normally precedes the tracking phase, but I see no reason to distinguish them.
- The majority of 'advanced' techniques that operators get paid (not a lot of money) for are importing, editing, and exporting. Again, workflow. Key commands. Having special equipment that can process, archive, render, and print files faster.
- The studios have massive multi-operator Pro Tools rigs assembled to servers that send and upwards of 6000 channels of audio across networked server systems, but at the front of that channel is the same stock Pro Tools digital equalizer that we all love for its ~cough cough~ 'vintage' sound..
- But the advanced 'mixing techniques' themselves. EQ. Panning. Bussing. Gating. Automation. EXACT Same stuff you and I do on our computers in our home studios. Just with a hell of a lot more channels.
So. What higher level mixing techniques are a 'MUST' for professional sounding stuff.
My answer is NONE OF THEM. Simply that perfecting the basics will go a VERY VERY long way. I could rattle off a list of complex techniques like verb transient imaging, envelope re-shaping, using voltage control simulators for automate soft synth behavior etc... but NONE of them are a 'MUST' for professional sounding stuff