Hi YDX6203
At long range, inconsistencies in your ammo (and technique) really start to show up, so anything you can do to make things more consistent will help.
With the brass, this would include length trimming (personally I do this just the once, others do this every time), inside/outside chambering of the case mouth (to avoid scuffing the bullet heads), and the steps set out above. At long range, particularly if using high pressure ammo, then FL sizing is the way to go (I don't know a single person who shoots competitively at long range who only neck sizes; at least with magnum cases).
You can resize your necks using different bushing sizes to adjust neck tension. However, unless you're neck turning (which is another discussion, and arguably pointless in unless you're using a tight chamber) then my view is that neck bushings just push wall irregularities inwards and won't help with consistent neck tension. Expander dies are OK but they're hard on brass, and will cause the necks to stretch... if you can remove the expander from your die and use a separate expander mandrel, then I think this is the way to go (you can buy different size expander mandrels from 21st Century shooting, and perhaps elsewhere). In terms of measured seating force, I've found mandrels to be superior to neck bushings.
I tried weighing cases, sorting them by wall thickness etc. and never found this to have a measurable effect. Interestingly, I've never seen an effect related to case or loaded bullet concentricity (within reason of course) so I've stopped measuring for this. Saying that, you can improve concentricity by seating a head half way, rotating the bullet half way, then resting the rest.
Needless to say that powder charge and seating depth must be consistent. Variations in velocity will be very apparent at 1000 yards. If you can get real world extreme spreads of about 20 fps (tested across more than 3 rounds ) then I think that's fine. You would generally try to weigh charges to the nearest kernel. I've found that primer selection can have a major effect on ES - this was a surprise to me.
If your bullet heads are already tipped, then your sorting options are limited to weight batching and base-to-ogive sorting. You can also sort by bearing surface and OAL, but I never bothered with these. If you're shooting a traditional copper match head, then you can trim/ uniform the meplats and/or re-point the meplats. Personally I have found this to make a big difference in terms of vertical spread.
Depending on the number of firings you can look into case annealing also. If you take the plunge, then I would suggest annealing after every firing... again, to keep things consistent.
I think that's most of it. Watch out for your technique also, as your point of impact can change considerably based on how hard you hold/ load the butt. Don't let your barrel get too hot.
But the killer is the stuff you can't control, predominantly the wind.
Best of luck!