The small mistakes I made learning from books and doing myself don't come close to some of the B/S I've seen on the internet.

Ditch the expander ball and use mandrels (or Lee Collet).

I feel like I shouldn't have to ask, being a reloader for 25+ years, but how does doing this in 2 steps with the same die rather than in one step with said die, end up in improved ammo? Am I missing something? 🤔

Keeping accurate records. I tell people just getting into reloading to write everything down in a ledger. Caliber, bullet info, powder, primer, case info, number of rounds, dies used, reasons for that load, date, year, intended gun, etc. I also add results , good or bad, after I've had a chance to try the load. Each load is assigned a matching number and info sticker on the box.
My first reload was lot # 38-1 shooting 125gr jsp with 4.4 gr of bull's-eye 5/10/1972.

Gives me something to work with. For example, I've got three rifles chambered in 44 mag. Each one seems works better with something a little different. Sometimes the loads are just for a different purposes, plinking vs hunting loads. Between revolver and rifle load I've got 15-20 different 44 mag loads on the shelves.
Working up an accuracy load the book goes with me to the range so I know what I'm shooting.
Had to move my gun shop recently, found stuff I reloaded 30 years ago. I'm in the process of shooting up some of the old stuff now just to clean up my inventory, dug out my old books to see what I was shooting.

Properly executed in two steps the case is sized on the up stroke and the neck expanded in same fashion. Do not use a full stroke with the expander. Just enough to pass through then withdraw. Doing so minimizes case distortion the asymmetric forces associated with the shell holder geometry cause.

Toss that collet thingy that comes with an impact bullet puller and use a shell holder instead. Faster and easier.

Whoah, really? Gotta try that.

Never have more than one powder container open on the bench at a time.