Trying to be ready for events when you don't know the exact requirements is a waste of crystals. If you refresh hard nodes for a character that isn't needed for an event, you have just wasted crystals starring a useless toon. Even if the event just gets delayed, those refreshes could have been spent on gear.
OP asked for most efficient farming strategies on limited crystal income, so unless you are at a place where you only need to farm newly released characters/ships, you should be targeting events with known requirements.
Therefore, it is most efficient to not refresh hard nodes. Instead, determine how many shards you need and what hear to farm, and grind it out.
I have gotten every legendary/journey/epic character on the 1st or 2nd pass of the event.
Hoard gear and crystals and spend them only when you are close enough and need to make it in time for an event. That is the most efficient way to manage resources.
