You had untreated early menopause for 8 years. You are at a very LOW risk of endometrial cancer.
An endometrium of less than 11 is also extremely unlikely to be malignant.
Endometrial cancer typically presents with chronic, irregular bleeding, not a one off withdrawal bleed triggered by missed progesterone.
It is good to get checked but you really have very little to worry about.
There is a completely disproportionate, literally hysterical

reaction to bleeding on MHT within the NHS. This is a common side effect and almost never malignant.
It doesn't warrant a "suspected cancer" referral as no menopause literate doctor would suspect cancer clinically in your case.
This results from the system being unable to distinguish between a postmenopausal woman not on HRT with unexplained bleeding, which raises a higher suspicion of malignancy and really does need urgent investigation, and a woman on HRT with a recognised side effect who could be investigated routinely without all the drama, because until very recently only 10% or less of women were prescribed MHT.
As much as we would like to think there is a set dose of progesterone for each dose of estrogen, in reality we are all so different that what suppresses the lining in one woman may need to be doubled or tripled in another to achieve the same effect, even though both take the same estradiol dose. It's like shoes - some people need a size 3, others need a 10.
Particularly this is the case when the estradiol is taken through the skin - there is such wide individual difference in absorption (more than 10x) that two women could use the same amount of gel or the same patch, and one would have a really good level of estrogen like a menstruating woman and the other could have undetectable levels. The first obviously needs a robust dose of progesterone, the second is taking progesterone pointlessly (and probably wondering why she feels like crap).
If you double the progesterone between now and your investigation, you may well be pleasantly surprised with a nice thin endometrium by then.