And should I roll this 30FG banner ?
Totally yes. That's the cheapest way to get 6*. It also gives a lot of Rainbow Shards for 6* promotion.
As I said, Snow Drop New Year may be the best choice here since she is not in the regular gacha. She may not be the best but she is the rarest.

Honestly I'm a fan of emity-like unit (Cygames term, low HP = stronger unit), does this type of unit exist in this game yet ?
There is girls with more HP = more damages but the only conterpart may be Dragon Tree (in a certain way). There is also some girls like this Wolf Berry who deacrese HP and increase power when below 50% HP. but I don't like them. On Castle, you want to stay at 100% HP. On the other stages, you want to be above 100% HP with HP retain because bosses hit too hard.
They may just be hard to play but I don't think this kind of girl is really strong anyway.
Dragon Tree looks cool anyway.