In my experience a lot of synth patches are all over the place when it comes to level. So I suspect that your first task would be to get the songs together, then check the average volume level for each song and the peak level for each song, and try to get them somewhat consistent. Then what you feed FOH won't vary by much, except possibly for the vocals. If your levels are reasonably consistent, you shouldn't have any problems with FOH.
Obviously if you need echos and verbs, electric or acoustic percussion and such, it is your responsibility to provide that.
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