Fred:
I don’t know how much money you want to put into it but I would like to mention some alternatives to iZotope.
There are 3 + 1 “main components” that determines a mix and a master (besides instruments and effect plugins like distortion, filter, flanger and alike). The 3 are
Panning
EQ
Reverb
Think of a mix as a room where you place the instruments. Place them so you can “see” all of them, non blocking another.
Panning to place them left to right. EQ to place them up and down (treble to bass) and reverb to place them back and forth.
Reverb should always be the same for all instruments to make a “in the same room” feeling. When I say “always” there are of course occasions when you want to deliberately deviate from that as a special effect, but then it should be by determination and not by chance. So reverb is something you set up as a bus and use “send” for (saves a lot of cpu too) while the other two are placed on each track.
For panning Cubase is, to a 99% degree, more than good enough. (slightly cumbersome if you want to record panning manually).
For EQ sound quality I find Cubase to be good enough. The GUI is however rather cumbersome. I prefer the EQ from FabFilter.
For Reverb, there is a matter of sound liking and GUI. My preferred ones are: IR live from Wavelabs, FogConvolver from Audio Thing and Valhalla vintage reverb. The two first ones are so called Convolution reverbs. For all practical reasons that means that you use more or less actual reflections that someone has recorded, and there are a ton of reflections to download for free or bought. Altiverb has the status as the “king of convolvers” but I have noe personal experience on that one. There are good reverbs in Cubase too, even a convolver, but I find it too cumbersome to use, especially for importing reflections.
The +1 is the compressor.
Think of the compressor as an “energy” facilitator. You can increase or decrease “energy” for each instrument in the mix with it. The compressor is perhaps the one of the 4 components mentioned here where it is easiest to hear differences between all alternatives on the market. I prefer Soft Tubes Tube-Tech CL 1B, but there are others very good ones too, and bear in mind that I rarely do EDM music. I am sure I would prefer another if I were.
In the master the Compressor is what gives you “punch” (think of a boxers energy). A multiband compressor could be of great use for some tracks (CL1B is not but the one in iZotope is and the one in Wavelab). It is the EQ that makes it “shiny”.
For mastering, a Limiter could also come in handy, especially if you are into more modern music, like EDM. The limiter has sound qualities like a compressor (in fact it is a compressor with a roof top limit). I prefer the ones from IK Multimedia.
For all pluggs “subtle” is normally more than enough, unless you are looking for special effects.
Dont take my word for what is “best” but perhaps to use them as a kind of starting point to find the ones that you are comfortable with. There are a lot of free stuff, but they normally don’t match up with something you pay close to 400USD for. No rules without exception and the ones from Voxengo get honorable mentioning. Many plugs give you a “try before you buy” so just go ahead and dive into the jungle!