My mates all think my bike packing is OTT. But, from memory….
rotors off wheels.
frame/fork spacers to stop them getting crushed
rear mech off, wrapped in bubble wrap & taped inside rear triangle
chain shifted onto big ring to protect teeth
handlebars off & taped along top tube.
Whole thing wrapped in copious amounts of bubble wrap/grey plumbers pipe lagging using masking tape and/or large re-usable zip ties. Take enough tape so you can re-pack at other end.
I also had a large block of dense white packing form that I cut into shape so the BB rested on it and supported the bike off the floor. A simiar piece supported the rear dropouts & the fork dropouts.
I used a Brand-X bike bag lined with a cut down bike box for rigidity & protection that I got from Halfords.
If you are taking pads, you could wrap them with the bike as a bit of extra protection.
Spares – depends where you are going really and the availability of parts.
I would take:
rear hanger
a few spare powerlinks for the chain
brake pads
Brake bleed kit if you have one (but that was due to paranoia over tempremental Avid brakes.