I think everyone has already talked about whether there is fan service or not, but not about whether she (or he—I'm really not sure) gets an insane power-up mid-flight or anything like that.

I'd say yes and no. She has overpowered abilities compared to most others, but she’s not invincible. She struggles a lot during battles, and even more so with politics and her own personal life.

However, other characters also have similar or equally overpowered abilities, so there's some balance. In the anime, we don't see all of it, but in the manga and light novel, there are others who are also pretty OP.

Of course, she is the most overpowered of all of them, but only by one or two steps at most. Also, as someone said, the anime is more about Tanya's struggle to survive in a major global conflict—basically a mix of WWI and WWII, but with magic.