Wow thanks for the extensive and interesting info.
As you say, I was thinking about the usual reverbs and delays, but in Eurorack module format. I have a few stomp boxes for the guitars but I wasn't thinking of using those.
The setup I have in mind when I mention FX is really a simple one. My audio interface has 8 inputs, all of them with pad switches, and for now I have no problem sending audio signals directly from the Eurorack modules into it. It peaks in the region between -18 and -12 dB at unity. So what I was thinking really was just to send mult'ed copies of my audio outputs into a matrix mixer, then from the mixer outputs into the FX modules, and from them either back to a simple (sub)mixer or directly to the audio interface.
That's where I have the Doepfer matrix mixer in mind, but I wasn't sure if I would be doing/buying something unnecessary, or if instead I'm being too traditional and there are better more modern methods.
In any case, I hope this is of interest to the OP too. I'm not trying to hijack anyone's thread...