Go through the wiring diagrams and highlight what wires you need to keep and those you don't. Go through each connector and de-pin everything. Throw away what wires you want gone and wrap and shrink the wires you want to keep in the harness. Then re pin all the wires back into the connectors. If you are looking for a list of wires to cut from us - you won't find it. Everyone's list of things they need in their race car is different. The rally guys need horns, windows, blinkers. The TA guys need none of that crap - not even headlights.
Someone up there said they have a harness in a box that it ~40lbs... I ripped a complete harness with all junction boxes out of a wrx a few years ago and it was not that light. I could barely carry it it was so awkward and heavy. I would say 55lbs+ for sure. If you want to take it to the next level, go for a PDM and eliminate the fuses and relays. But judging from the question you asked in your first post, you might not be ready for that type of system.