Should I Spend Crystals On The Vets?

So I currently have both vets at 4*, I have been preparing for around two months or so for the rest of the characters, including the First Order for BB8. I don't know if I have the time to get them to 7* however since it's about time for Rey's event and BB8 is starting to show up on the ingame upcoming events. I am willing to spend money if it's impossible to farm within the amount of time, but if I do I want to spend the least amount of money possible. I am already doing 3 Cantina refreshes each day since I save my crystals and occasionally buy packs. Other than that though, are there any other tips to minimize the amount of time to get them to max while at the same time being efficient with what I spend? Also are there any packs that come to the store where I can buy a guaranteed amount of shards for a good price?(Not those 5-330 shard scams where they have 5 shards at a 99.9999% drop rate)