Would be amazing to have the option to return your FX to processing channels.
IMHO you can do this already (if I understand your request correctly).
Go to the FX, choose backpanel and you can route the output anywhere.
Please do correct me if I’m wrong!
Yes, unfortunately you are “wrong”.
At this point, you can set (in your example) “FX1 Snd”, which is the source for the input of FX1 Unit.
To set the destination (output) of the FX1 Unit, you would need to use FX1 Return.
It’s not entirely clear what the OP means by “processing channels.”
If, as suspected, he means input channels, then unfortunately it’s not currently possible.
And I think, that’s why he made this request.
However, if he would means mix channels, then this has always been possible, but not on the FX’s back panel, rather via FX Returns in the respective mix.