Frame similar to small Sonder Frontier with lower stand over

After 2 hours, my Google ability is waning, much to the disappointment of my better half who expects me to come up with bike related answers in minutes at most.

Looking for a frame to become a bikepacking type thing for her. She has a gravel bike but isn't happy on looser and rockier terrain and her FS mtb is not practical as the saddle pack thing doesn't work.

She's only 5 foot 2 and whilst the Sonder Frontier small frame fits geometry wise, the standover is about 60mm too high.

Does anybody have any recommendations for a suitable frame up to around £600.

Doesn't really need the luggage frame material etc, but would like something for 100 to 120mm forks and ideally 27.5 x 2.8 tyres.