its crazy to me that Flo is used so widely, as its started by Russian men and their treatment of data has bee public for a while, it just hasnt spread fast enough. I know theres at least one other option called Calessa (http://Calessa.app)
There's a whole heap of different period tracking apps these days. I've been using Clue for probably a decade.
That one is good I think. It's German and adheres to EU privacy laws. The main FLOSS one is called drip. Has some funding from the German government as well as Mozilla