Everything apart from:

– Wheel building
– Headset fitting (though I might just buy a press and do these myself)
– Facing

I'm trying to learn how to service forks and shocks so that I can score that job off the list – first faltering steps seemed straightforward.

It's immensely satisfying to build up a repertoire of skills over the years which can be combined with some basic tools to save a lot of money.

(BTW – brake building/bleeding is very easy, despite being messy as ****)

Nothing and never bought a frame that needed facing.
Most stuff on bikes is easy as it is generally just a matter of bolting it on and aligning it correctly.

would you consider using a bike tool co-op for those low use high cost tools?

In theory yes, but in practice probably not.

Same here. It's a nice idea but these things need doing so rarely it'd probably be more convenient to just take it to a shop.

rear shocks
building wheels – although I have replaced some rims
full fork service (bushes etc)

MisterP Re. XTR pedals, you need to hold the pedal body still (not the axle as all that will happen is it will all just spin on the bearings!) and turn the part mentioned, which is ally not plastic and has an arrow on it to show thread direction.

Facing post-mounts helps to ensure the caliper and pads are true in all directions, the mounting slots will only allow adjustments in one plane to centralise it on the disc.