Re mushrooms for ER+ disease, white button mushrooms and other common supermarket varieties (not maitake mushroom) have been shown to reduce aromatase (the conversion of androgens to estrogen within the body) and are safe. White button and other mushrooms have been shown to reduce proliferation and induce cell death in ER+ cells. Also, Asian population studies have found a lower risk of breast cancer among those who consume mushrooms frequently. I am not suggesting mushroom supplements. Below are some supporting studies that you can Google if interested:
Anti-Aromatase Activity of Phytochemicals in White Button Mushrooms (Agaricus bisporus)
A case-control study on the dietary intake of mushrooms and breast cancer risk among Korean women
Dietary intakes of mushrooms and green tea combine to reduce the risk of breast cancer in Chinese women
Dietary mushrooms reduce mitogenesis and induce apoptosis and cytotoxicity in MCF-7 human breast cancer cells