That was great!
Sometimes I just know right away. Otherwise, if I don't initially gel, I might put it away and try it again a couple months later. Honestly, I trust my gut. It you're still in the return window, you could return it if you don't feel good about the purchase. I remember with the Fractal Audio FM9, I bought it because I thought it was going to work great, actually learned some things working with it, but then quickly soured on a few things it can't do even though it promises that it can. I just got to a point where I kind of knew that it wasn't going to be for me.
Most things that sit unused for six months are candidates for de-accession, but I don't like selling things so sometimes it takes a lot longer. There are a few things that sit around even if I don't use them that much because I know they're worth having for "that thing" they do: Grendel Drone Commander, Moog theremin, Octatrack, etc. Since I'm Eurorack, there are also some modules that are only sometimes racked.
I've only had a couple small things I've regretted selling, and "regret" is a strong word there. Maybe a few years later a need comes up and I think "oh, that thing I sold could have worked for that."
Unless I'm missing something (?), that sounds like a pretty standard multichannel mixer setup. Each synth gets an input on the mixer and the pedalboard is on a send, with one or more returns to its own channel(s).
