This is certainly an important issue, and one which severely impacts the quality of life of many women, including BC survivors!
Although we do not yet have data on the safety, for ER+ BC survivors, of the use of locally (vaginally) applied estrogen products, two, Vagifem and Estring, provide very low doses of estrogen. Estring, for example, releases over 90 days an amount equivalent to taking oral estrogen for just 2 days, and Vagifem is now available in a 10 mcg dose, whereas it previously came only in a 20 mcg dose. (Both of these are reported to be preferrable to a similar third product, Estrace.) These products also appear to have a very, very low level of systemic absorption.
Due to these factors, oncologists are becoming more comfortable having women who have had BC use these preparations. (And as a previous answerer mentioned, this may not be a concern for you, since you have been treated for TNBC; but do check any new medications with your oncologist.)
Meanwhile, a clinical trial using Vagifen 10 mcg in women with BC is currently under way, and use of Estring is also being researched.
Finally,since you have had problems with cream and gel type lubricants, I would highly recommend trying Pjur Original Bodyglide. Although it works as both a moisturiser and a lubricant, it is oil-free, water-free, and fragrance-free,and not at all sticky or gooey. It can be used for massages as well.
Best wishes for obtaining improvement in regard to this issue...