One lesson learned so far is when resizing of brass, be careful on false measurements if you're only checking the dimensions of the brass post firing. F-Class John covers this in his YT videos as well as others. Where if you measure after a first firing, the case may not have expanded to the chamber. Then if you bump back the shoulder based on only case measurements, it can lead one to sizing back too far resulting in working the brass more, which can lead to case head separation and other stuff. This didn't happen to me, but a friend noticed problems with his brass, which I think started as soon as his third firing.
When I did get around to setting up my sizing die, I did it the way where you take fire formed brass and you bump the brass back and check in your chamber (with bolt extractor removed). So I verified my brass headspace to the chamber. In that process, I did notice that some of my once fired brass that was the first pass in my load development was varying case sizes. I'd have to check the measurements, but it went from some fired brass being where the bolt had resistance to lock to where the bolt would drop freely. The brass was already decapped, where the brass from that round of testing was mixed up, so couldn't check to see if the cases that didn't fully expand to my chamber was from the lower powder charges.