Honestly I wouldn't worry about this spotting at all.
You have been on and off HRT and had negative investigations after the spotting started.
Nothing you have said is remotely suggestive of endometrial cancer. It is far more likely to be an atrophic and unstable lining breaking down, or due to vaginal atrophy.
The uterus is designed to bleed and all the angst and scaremongering about "postmenopausal bleeding" is disproportionate, irresponsible and harmful.
I bet you don't even have any risk factors for endometrial cancer such as obesity, diabetes, family history, chronic anovulation premenopausally, unopposed estrogen etc.
You also have previous progestin use as a protective factor (on kliofem) and that isn't even taking account of any previous progestin used within birth control.
Less than one percent of women NOT on HRT who are investigated for PMB have malignant changes. Most probably never comes to medical attention so the actual proportion with cancer will be even lower.
For current or previous users of combined HRT this is even lower still.
I am on a combined pill continuously, have been for 17 years and about once every year or few years I get a bit of painless light breakthrough bleeding.
It doesn't faze me one bit because I understand the physiology and know that I am at exceptionally low risk for cervical or endometrial cancer.
I also pay for a private ultrasound yearly for ovarian screening due to my family history, so I know my endometrium makes Kate Moss look like Kirstie alley.
If you want reassurance without the trauma and mismanagement of going through the NHS you can do the same any time, I use ultrasound direct.