Just to add to what Tenhou said you should try to aim for getting over 1k pink keys because then you will only find boxes instead of boxes and keys (for pink). It does cost 1.5m pero though to reach but its definitely worth it... plus you get a ton of ngatcha tickets. Outside of that though there really isn't a good way to dump pero.

1) 1000 pink keys during gambit
2) trading N wild for R wild ( because after advancing in game you will have ton of N wilds through gifts so after maximizing all N cards this is good to do )

3)buying elixirs and energy drinks in EGuard event