My favorite mistake was planets that have hit 75 society level, but have rubbish population. Must have used those ones for arc-jumps early on.
Yup.
That's why I use my MC suffixes. I can always see which planets still have rising soc, which don't. When they hit 65ish, I'll start building MC.
Pop still grows after MC, so I should have every planet I own functioning at about optimal productivity eventually. I will then construct fleets of several hundred (or maybe thousand?) cruisers, and deploy them against Oldry. I might lose. But I will have loads of fun with it.