My mixer channels are a gain node (for input), followed by 2 gain nodes - which are used as fx slots and replaced when an effect is added, the last fx slot conn

My mixer channels are a gain node (for input), followed by 2 gain nodes - which are used as fx slots and replaced when an effect is added, the last fx slot connects to another gain node (for channel output) - that then connects to the mixer master gain node. You could have it as simple as 1 gain node for each channel which you use for input and output to the mixer master gain. This would make it difficult to add fx later though.