Unfortunately this is a case of treating female patients as if they are just a uterus rather than looking at you holistically and thinking about the top to toe effects of estrogen or its lack.
My approach like Sheila says would be use the dose of estrogen that you feel well on, and titrate the endometrial protection to control your lining on this dose, rather than stripping your whole body of estrogen to manage a lining that is not pathological in the context of hormone therapy.
It is worth remembering that the normative value for endometrial thickness of 4mm was derived from UNTREATED postmenopausal women.
No such normative limit has been agreed upon for women on hormone therapy, but if your skin, bones, genital tissue etc are plumper, stronger, juicier then it is illogical to expect the endometrium to be all shriveled up and completely atrophic.