Admittedly, I havent used the STM. I've got 9 switches and I just used a CSM2 because i needed the data driver capability. I didnt use breakout boards, and I dont have buttons. Im iPad/remote only. The 2 switches that dont fit in the CSM are somewhere that I can see if they're thrown right and its kind of irrelevant if they're not (runaround track).
Dennis Zander says you need 1 data driver, so it would seem to follow that you would just need 1 CSM2.
The only interface the DZ2500s really require with the CSM is via the blue data wire, the switch sense connection is optional, but I understand thats the whole point of having the module.
I dont understand why you need relays and stuff, The relay should just be for auto derail activation on the switch. If you dont use the relays for auto derail (DZ2500s are sprung and you can run backwards through them anyways), it looks like a matter of simply connecting the green or yellow wire (whatever makes it read the right way) to the STM2 and grounding the return to the outer rails on the track. I'm willing to stand corrected if I'm wrong.
I'd be curious to know if you use multiple CSMs, if you're connecting each data driver or just running a data wire around the layout and connecting them all to a single CSM. The address of the CSM is linked to the switch sense positions, so you can give a bunch of CSMs different addresses and the sense positions would all work without connecting all the data wires.