What ivs would you keep?

96% and above for living dex, and also all legacy movesets or if multiple of same legacy moveseg, keeping highest IV.

Well it depends. Legacy but mega-relevant mons like Lick/SC Gengar, MM Metagross, SD Ttar, SB M2 are worth keeping even with mediocre ivs (Gen 4 could make them less worthy though). For dex fillers personally I would always keep the highest iv and roll them out whenever I got a higher one.

Mine is pretty close to yours, though there are exceptions for rare mons, legacy movesets, etc.

87+ with 15 atk and 93+ with 14 atk. I only keep the 13s on 96s that are good defender mons.

Also: just keep the very best two, overall for dex fillers (a Male and a Female).

I look at it a bit differently, I decide how many of a species I'll keep and that will set the threshold IV. If I happen to get more than the decided number with great IVs I'll consider again. For most species (those that don't matter that much) I will delete anything below 98% without blinking.