I really like your mod, but there's a feature your mod definitely needs: either a dedicated wall-through block with either wire conductors or relays on opposite sides to conduct power through walls or floors/roofs, or the possibility to conduct power through a block by just putting conductors or relays on blocks on 2+ sides.

You can already do that.

You should read "Basic Wiring" in the manual again. It provides a sollution for this, in the form of "Feedthrough Insulators".

Oh, now it's clear where I went wrong!

I used relays instead of connectors, and I didn't think of tapping one of the connectors with the Engineer's Hammer.
(Didn't realise the latter until I read the page about the Feedthrough Insulator again just now and saw the animation, which gave me the clue about trying the Engineer's Hammer.)

Kudos for pointing me into right direction!