Hey there @andytheyoo
Wasn't sure where this track was going listening to the intro & first part to be honest, but then came the break. Nice sounds in there and the second part works better than the first one IMO. Sounds are more defined, it's less cluttered & busy, more pleasant.
Yes, same here when listening to mono, the mix became thin and muddy, especially everything synths & effects, the drums retain good mono compatibility. As for answering your question for the synth sound design & process, it's a tricky one I must say. As soon as you start to use Unison the sound will spread out a lot. Using 2 oscillators with less voices, one OSC panned L and the other R can help (if your virtual synth allows to do this).
M/S eq is also quite tricky to setup. It's a very good technique but not that easy to nail & to get right, it can drastically impact the mix, you need to make gentle changes in the EQ curves, boosting a small amount of db on the sides and targeting the right frequency area but with a broad Q factor. Boosting too much with a narrow Q can make everything sounds harsh and bring unwanted resonance.
You also have a noise layer ( not sure if this is part of a synth sound or a layer on it's own ) that takes a lot of energy from your synth, could be filtered & attenuated in volume IMO.
Some people still prefer to mix in Mono first and adjust the stereo later on during Mixdown stage, some really don't pay too much attention to Mono anymore since more & more playback systems aren't just Mono but Stereo. So that's all arguable in the end.
Not sure if that's answering your question but again , it's a tricky area, not that easy to get right