it was clear as mud
you said... "I do not want to assign these "effects" to channels, because I do not want to affect the main mix, only the "monitor mix"
That was hard to understand because you don't assign effects to channels, but you can assign channels to effects.
There is only one reverb "effect" and just like an aux mix, you need to "turn up" whatever input channels that you want in the reverb.
Each of the six monitor "Aux" outputs have faders for all the input channels, including the reverb and delay channels.
If for instance, Sally is mix one and she does not want any reverb in her mix, just leave the "reverb" fader all the way down in her mix.
Each monitor output does NOT have the ability to have sa different reverb.
You mentioned "effects such as feedback suppression and EQ"
Feedback suppression is not an effect and the DL 1608 does not have any feedback suppression.