Hi @Tangram
Welcome to this wonderous world / universe.of Zynthian. I hope you enjoy all it has to offer, not just the device but the friendly community too.
There have been a few posts in this forum about balanced inputs and outputs, e.g. this one announced their arrival and discussed their configuration.
The inputs are configurable which is quite cool but outputs are fixed TRS balanced. This is not an issue though. Connecting a standard TS mono 1/4" jack to the Zynthian will carry the output signal just fine as if it were single ended (unbalanced). You can use mono TS cables to connect your Zynthian to your mixer without issue. That is probably the most common configuration of Zynthian users.