Primers: what are Benchmark primers? Benchrest? For 223 in an AR, I like CCI #41's. in the current shortage, I would use any Sm Rifle Primer. Primers don't matter much in an AR, except in preventing slam fires which the #41 does best.
Powder: I would not use surplus powders. I would use Benchmark, CFE223 and the like. VARGET does not meter well, but is a good powder. It also give compressed charges...which sometimes overflow the case neck. This is why VARGET is not used by me.
Do you have bullets? If not, I would hang this project on a hook for 3-6 months.
Your trimming sounds excessive, but fine.