Holy moly, I knew Eyja did higher damage but I didn’t realize it was by that much. And since that’s just base stats, I assume it’s not counting that her S2 does over triple damage. What site is that btw?
To answer OP’s question, I find the big 6*s worth savings for are Eyja, Saria, and Exusiai - in that order unless you have good replacements for them already (Silverash is awesome too but from the banner list it looks like he’s ever been buyable once on either server, so don’t hold out hope). All of the others that come up on the store are good buys, but if you have decent alternatives to them (i.e. Texas for Siege) then it might not be worth it. Depending on how new you are and how many roles you have filled already, it may be a better use of yellow certs to get some good 5 stars.
On the other hand if you’ve been playing for awhile and already have a good lineup, the math works out better to spend it on headhunting permits and use them on banners that have operators you want.