Onsen in Japan as transfem

I know there are many posts on this topic but none quite get to a satisfactory answer for me.

I've been happily going to onsen for decades all over Japan. I've always gone to mens side and don't really care about going to the women's side (agender). I'm also still early on HRT and no surgeries or plans for any except maybe FFS in the distant future. I don't pass. Long hair is easily pulled up. Gender neutral clothes can be worn on the way in.

At what point do I need to stop using public onsen? Is there some amount of breast development that won't cause trouble? When do I know the line is crossed and public onsen are no longer available to me (as sad as that makes me). Is there a cup size? I suppose that it's too blurry a line to have a hard and fast rule most likely. Should I just not worry about it until my first male fail? I'm not worried for my upcoming trip and plan on savoring those onsen visits as if they're my last cause they probably will be 😢

fwiw I speak intermediate Japanese and would feel comfortable asking management if I knew the right way to broach the question if that were helpful at all.