Treatment??

It may depend on your age and quality of life. There are different chemo's available to slow it down and radiation to manage pain. If you are game as its not legal yet, you can even do cannabis oil too.
Just Tamoxifen sounds like not a lot of treatment but perhaps it's a plan to see how well it works in staving off advancement of the mets? For some the results can be very good, for others, they need more.
There is a uterine cancer risk but honestly its a very low percentage of odds and it doesn't just come all at once, its a gradual thing and even then, it's got treatments too.
At least Tamoxifen isn't as damaging to your bones as others.