#The following is based on my personal research and experience as a family member of a breast cancer patient - please consult your doctors/specialists before making any decision#
If you have had family history of breast cancer - or other cancer - on your mother's side (aunts, grandmother etc.) you can benefit from genetic testing for the BRAC1/2 genes. If I understood right you have checked and that's not a problem, so you are trying to deal with future uncertainty given that you are of higher risk than the 'average' woman.
A few things to note:
Tamoxifen is very good at protecting against ER+ breast cancers only. It will not address any endometriosis problems you have, and while it works for the overwhelming majority of women, there is no way of actually _knowing_ it works as intended. The side effects are usually tolerated so given the risk/rewards it may be a good idea to discuss it with your doctors. You do not need to have a period to be on tamoxifen, that is incorrect. You need to be aware that tamoxifen increases your blood clots risks and endometrial cancer risk.
A more heavy handed solution to address the risk of ER+ breast cancer would be to shut down your ovaries, either medically (e.g. Zoladex injections every month) or through oophorectomy (someone mentioned hysterectomy - that would be unecessary, an oophorectomy will do). Of course, you'd need to consider whether you still plan on having children and whether you are ready to enter menopause with all it's side effects. Shutting down your ovaries in any way will trigger immediate menopausal symptoms. They do get better over time (a few months down the line), but they can be hard to tolerate initially. The benefit of medically induced menopause (i.e. zoladex injections) is that you can stop at any time and will usually return back to normal after a couple months. Again, it's an option to discuss with your doctor(s).
However, besides any medical treatments you need to ensure you have the right screening in place. That can be mammograms and/or ultrasounds. Depending on your breast tissue density you will be directed to have one or the other, usually annually based on current NHS protocol. You can always opt to have both, every six months - if you can afford it privately. The most conclusive screening method remains an MRI with contrast, which again you could opt to do privately if you are very concerned, once a year. Physical examination by hand is also highly recommended as often as you can - many breast cancers start as lumps you can physically feel. Most times a lump is nothing of concern, but the earlier you go check it the better you will feel.
You mentioned family history of breast cancer but did not mention what type - that can have substantial impact on how high the risk you are in is. It would be a very different case if the history is that of ER+ cancer compared to it being triple negative.
Finally, with regard to the comment about estrogen produced through body fat. That is accurate. Around 10% of estrogen is produced by body fat, which is the reason post-menopausal women are given additional medication (e..g letrozole) to reduce that part of estrogen production even further. In pre-menopausaul you will usually get tamoxifen and some ovarian suppression (e..g zoladex).
I hope it helps, but again, make a list of questions and speak to your doctors. Do not take no for an answer and do your own research on whatever they tell you. Ask others but make sure your information is factual from research sources.
