It really depends. Do you like her? If yes, then just go for her. No need to think twice. The longer you play the game, the more rare prisms you'll get, so eventually, you can get all servants from there. I'm sure.

Edit : For 8th anniversary ticket, just wait until anniversary event came in a few days. You can change those tickets to some beautiful CE.

While BB is useful, wait. One of the Pilgrimage campaigns adds NP levels and Coins to the store, and then you only need some Evocation Leafs

That really depends, are you actively using her as a DPS?

Do you need anything else from the RP shop, like say, Bond Grails?

When BB does get added to the Evocation shop, you'll get a refund of the RP you spend anyway. But if you're not using her a lot, and need Bond Grails, don't do it yet. Better to wait up.

Everything I've come across says you don't get a refund for spent RP on NP levels for BB and Santa Altera. So please be careful about confirming is correct advice first.

I find that hard to believe. Everything in the RP shop has the refund clause attached when they become available for free.

I'll try to look it up somewhere.

Edit: I stand corrected, they clearly said there will be no RP refund. Thanks for pointing that out @DeltaBladeX ~

So yeah, don't exchange the NP levels rn, especially if you're not using her a lot.

They didn't refund them here because people could have multiple servant copies. Those RP are gone once spent. I was pissed, since I did so just a day before Santa Altera was added in Japan.