Hey! Thank you!
No need to worry about money balance. I testing the game long-enough every update, so I have time to think through the economy thoroughly, without getting too stuffy and boring.
Mira has a multiplier of relationship growth (x1.5), and yeah - level up a relationship with is very easy, this is probably how it was intended.
As for jewelry, no forces you to buy it, and it's primarily needed by characters like Lisa and Dana, where increasing their relationship level is crucial. Mira's relationship level increases quickly anyway, so buying and gifting it to her is an option, but it's not mandatory to use it.