This is more of a reloading question, but the reloading guys hang out here to.
For hyper accuracy, I've not had luck with 1:8" barrels with varmint weight bullets.
Lighter weight bullets often like a slower twist rates.
The .223 family started off as 1:16, 1:14 and so on. I spend more time finding accuracy nodes with faster twist barrels in the 45-55 grain bullet range.
For a very reasonable price varmint bullet, I've been using Hornady Vmax which has really stepped up it's game with these bullets. Remarkably consistent, and the size/weight advertised.
Most of us are aware of Win 748 that is dirty, heat sensitive but will deliver some groups to brag about.
I've switched to Benchmark lately, as good of results a 748 in most rifles, not as hard to clean & not temp sensitive.
As for brass, that's up to you.
Once fired brass is often about your re-processing/preparation as anything.
Starting with Milbrass let's you try different things, learn about preparation without costing you a bunch of money since it's everywhere and cheap.
You can put 10 round groups under a nickel using Milbrass, the brass often isn't the issue when your groups don't tighten up...
One tip is to buy Milbrass that is LC headstamp, 12 or newer (2012), these are most consistent and exactly the same shape/volume as commercial .223 brass (once the primer crimp is removed, a one time operation).
With Benchmark I find accuracy nodes between 25-26.5 grains with 50-55 grain Vmax bullets.
Depending on firearm & primers, usually well under MOA with a reasonable barrel, the ARs with fast twist barrels it usually hovers right at MOA or slightly larger.
I don't know why, but IMR often will show better results when I can't get Benchmark to do MOA in fast twist barrels. When you consider composition, it's against everything written, but it does seem to work when 748 & Benchmark just doesn't quite do it.
Another option might be a slightly heavier bullet.
60-62-65 grains often tighten groups in fast twist barrels when you just can't find an accuracy nodes with 45-55 grain bullets.
Load trials are just that, trials with every combo you can think of until you find the load that particular rifle shoots well...
This is the part that a lot of 'Shooters' hate, they want to shoot, not load, but I find it a puzzle to solve!
Write down EVERYTHING in your load book!
Don't skip anything, some minor process might be the difference.
Change ONE thing at a time, don't make two or three changes at once since you won't know what made a difference...
Hope you find a sweet spot, nothing quite like the feeling when you shoot sub MOA with a handload!