The only role long repop timers have is to make a rare spawn convenient for every region without oversupplying a NM. Make a 72 hour NM spawn at 8pm JP time, 8pm NA time, and 8pm EU time every 3 days. Half hour window, then it's over. Then everyone who wants a shot at it knows when it pops, has a convenient time that it pops, and it's still rare.

The hardcores will hate this, because it's convenient enough for the player base to beat their asses with.

You can have 24 hour public claim NMs. You just have to make them as convenient to camp for 90% of players as it is for the 10% poopsockers.