Drill another interim hole and keep the disc that falls out, use hole to assist routing, cut a rectangle of wood: width the diameter of the hole, length 1.5x the diameter. Partly wind a screw into it centrally to use as a handle, fit wood by pushing into hole and aligning. Use wood glue to fix and leave to set, next day use screw to attach circle cutout. Fill edges with some filler, sand and paint
Also, Aldi had inspection cameras the other day.. Kinda like a camera on a bendy rod, hand held screen. No idea on quality, but may suffice. I've also set the iPhone on video mode, enabled e light and pushed the camera part into the ceiling hole before now, keeping the body of the phone in the room to look at the screen. It's helped, if only to confirm I've completely missed putting a wire in. Yes, insulation filler makes the job a bit harder, but you might be able to poke it out of the way...
