Looking down my bookshelves:
Another set which my dc liked but aren't generally in print are the Monica Edwards books. They were marketed as pony books, but other than the first aren't really. There's generally boys around, but the girls are very much the leads.
However the prices can be quite high. I'd recommend "The White Riders" as a good start, but that's one of the ones that prices have rocketed, so maybe "Summer of the Great Secret" is a bit cheaper as a start off book. You may find some at a second hand book shop a bit cheaper.
Little White Horse-by Elizabeth Goudge is another good girl lead.
A Spoonful of Jam is a WWII story. It's the second in the series but the one based around a 12yo girl and reads well on its own.
Some of the Arthur Ransome stories have good girls in them. "The Picts and the Martyrs" is probably the most girl heavy as it doesn't have the Swallows in it, so John isn't there to take the lead. Also "The Big Six", where Dorothea is as close as any come to taking the lead.
I think some of the Eva Ibbotson books have good female leads too, but can't remember which ones offhand.
A lot of the Noel Streatfields have female leads. "Thursday's Child" and "Far to go" have a feisty girl at the centre. The Gemma series, if I remember rightly, the only boy is Robin who is the youngest. "The Painted Garden", "Wintle's Wonders" and "Curtain Up" (I think recently released as Movie Shoes, Dancing shoes and Theatre Shoes) all have girls as the main characters, as do several other of her books including Ballet Shoes. I think generally if there is a potential boy lead, they're often a twin of the eldest girl and away for boarding school for part, and fairly equal even when they're there.