Closing in on completing a set can be very satisfying, but soon after filling the last slot, I usually develop a case of upgrade fever.
One of my sore points with NGC is the way they define "sets". The basic Walking Liberty half set includes every date and mint mark, plus extra slots for the major design change moving the mint marks from obverse to reverse in 1917. This yields 65 slots, that was defined as the "set" when the Registry was created and remains so today.
The basic Morgan dollar set, however, includes all dates and mint marks, minor design changes (7 tail feathers or 8 tail feathers, straight or slanted arrow feather), and several very obscure errors. The minor design changes, OK, but I don't believe an 1882-O/S or 1887/6-O belongs in a basic Morgan set any more than a 1946 DDR Walker belongs in a basic Walker set.
Even more frustrating is when the set changes long after it is added to the Registry. A new set for Walking Liberty half dollars including 14 specific varieties was added years ago. I asked at the time why those specific varieties were included while others were not, and the answer was that those were considered the "significant" varieties fir the WL series. I already had a few of the varieties and immediately began searching for the others. It took about five years to accumulate 12 of the 14, but when I went to add my 12th one, I found two new slots had been added to the set. More slots have been added since then. To add insult to injury, I had preciously owned one of the added varieties but sold it because at the time (1) the variety had not been attributed on the label, (2) NGC would not attribute the variety because it was "not significant", and (3) there was no slot for it even if it was attributed.
That's my rant for today. Someday I'd like to get some of those Zimbabwe multi-trillion note you are collecting. I have some seriously devalued Ethiopian money but their inflation has been mild compared to Mugabe's pillaging of his country.