Hi there @StefanA
Honestly I think that full Mono compatibility is less relevant than in the past when most of the clubs PA were only mono, it was something that producers had to pay attention to when preparing their tracks for club, it's less important now.
It's enough to retain a certain amount of low frequency, especially for kick & bass, you can do that with a Utility tool inside Live, using Eq in Mid/Side can also help.
When it comes to the Korg M1 sounds & presets ( and many other soft-synths ), one thing to check is the effects & processing used directly inside the plugin. If you completely disable effects like reverb, chorus..etc inside the plugin, you'll end up with a much more solid & mono compatible sound. The counterpart will be that it will also sound quite thin in the mix. One technique is then to send the synth channel output to a return channel & process it with your own effects, or simply copying the track and layering the unprocessed sound channel with a processed one with your own effects and then blending them together to taste.