A limiter is the most brutal form of a compressor, you should use one (at the very end of the FX chain) but I wouldn't let it do too much, it can really start to sound annoying quite quickly. Before it, try just driving the present compressors a bit harder, i.e. any of higher ratio, lower threshold and lower attack. And of course gain makeup – you did actually check that the signal does go to the allowed maximum? Note that individual tracks may in fact exceed the limit, not a real problem if there's a compressor and limiter in the master track.
There are a lot of more special things one can to in dynamic processing:
- Try putting two (or more) normal compressor units in a row. Thereby, you can get higher total ratios without too obvious action of any of them.
- Lookahead. That's a feature of many digital compressors which allows them to remove fast peaks far more rigorously without need to set the attack so low that it sounds bad.
- Output level detection (also something the Cakewalk compressor doesn't seem to be capable of). This allows the compressor to actually control the output level, rather than simply react on the input level; usually performs better in terms of master loudness.
- Multiband compression.
Hang on here: before starting with that, you should make sure you've also set the equalizers really well. Specifically, from what's visible in the picture, there might be a lot of low-frequency junk around, which may substantially raise the levels without adding anything useful to the sound. Don't be afraid to use hard highpass filters! And try boosting mid frequencies that are "loud" and well-comprehensible but won't raise the signal level too much. Ok, where were we...
- Multiband compression. You probably didn't want to say too much of the bass goodbye, then these bass frequencies will not only raise the output levels but, of course, also drive the compressors, thereby reducing the level of all frequencies. A multiband compressor lets you compress the different frequency ranges seperately, so each can be maximized on its own.
- Nonlinear distortion. That's right! It's the oldest trick of all: "if that guitar amp won't get me loud enough inside the clean range, I'll just drive it over the limit". You can get a signal much louder by simply clipping it over 0dB, but it will obviously have a rather strong... influence on the sound. In particular, digital and in general OP-based electronics peak very harshly, resulting in rather unacceptable artifacts. That was different in "old" analog equipment, which saturates far more subtly. One piece of such equipment that is therefore still found in most studios is the tube microphone preamp. Rather more effective used to be the analog tape machine. The are today plugins which can simulate the characteristics of such devices, or simply give you the kind of benefits they offer. For instance, mastering multiband compressor and limiter tend to have such a soft clip unit built in. You still want to be careful with such stuff as it can literally destroy audio signals if done too strongly, but IMO it tends to sound more natural than directly driving a limiter too hard.