Lisa, I don't know about the clinic you use but my clinic in London insisted on regular transvaginal scans and it was a condition for prescribing progesterone ie no scan, no prescription so the safety side of it was a non issue for me. My clinic didn't like anything more than 4mm for women on a continuous combined regime. They also insisted on regular breast scans if they prescribed oestrogen.
My gynaecologist said that generally speaking, if you are using a continuous combined regime with inadequate progesterone you will bleed most of the time. If you use oestrogen only and never take progesterone (which is not recommended obviously) then your womb lining will either build up to a dangerous level or once it gets too thick, it will break away and cause a very heavy bleed - I know mine does this if it goes beyond 8mm.
God knows why pharmaceutical companies don't produce progesterone at a 50mg dose, it's so shortsighted. The doctor at my clinic also worked for the NHS and lamented the lack of choice.
It's worth pointing out that the compounded hormone route is a last resort for women who literally can't use any of the branded progesterone products and like yourself, are on the verge of a hysterectomy. It's by no means the first port of call for most women and most of us, myself included, would rather spare ourselves the expense and use a bog standard progesterone without having to endure life ruining side effects. For that reason alone, compounded hormones should be viewed more sympathetically.
For many of us the choice is stark, compounded progesterone or a hysterectomy which is major surgery.
I'm still trialling Darstin but I still don't know how well it's working but I'm happy to take the risk.
I really hope it works out for you.