I love
Lavinia, and I completely fail to see how it's frumpy. If one doesn't like it, okay, but I can't wrap my mind around the reason for the dislike being "frumpiness." I think for a name to be frumpy it has to have once been popular at a time when middle-aged and elderly women were being born, and
Lavinia has never been popular. It's ancient
Roman and very far from frumpy.
I don't think
Helle, no matter how it's pronounced, is usable in an English-speaking country.