The "not cutting" part is pretty easy. I used a vampire tap for that to handle switch illumination and would do the same here. It does puncture the insulation but you can remove it and just apply some electrical tape over the wiring in the future to undo it.
I have a 12-slot blue sea fuse block in the engine bay, though IIRC each slot has a 30A limit... which is probably fine for my purposes anyway but if not it sits directly behind a circuit breaker... though IIRC that is 100A which would be too much for a short on the longer run to the rear accessories.
I'm not sure about the relays.
@TeCKis300 posted the following in another thread. I would assume I could run pin 87 to a second fuse block but I'm not clear on how I'd run it to multiple individual circuits without using multiple relays.