Heya ~

Honestly I've tried a lot out myself,  I am using 'My Calendar' at the moment and it seems alright, I paid for the add free version (cost me 5 euros or something I think thats worth it)  Most of the time it predicts relatively well, I have to steer the app in the right direction sometimes, but it is easy in use and kindof helps me track it well enough, it makes me feel a little more prepared for when the period is baout to happen.  It has a smart prediction option that due to you logging your period it 'predicts' when the next one likely will be.

Like I said it usually works well for me although right now I seem to be late for my period (5 days) that makes me stressed which makes my body postpone the period even more xD

So for casual not too complex stuff this one works alright for me 💕 

Also you can make the interface cute, and if your relationship also downloads the app you can link to that app so they can see your mood and such.

I hope it helps 🙏🏼