There are no double-blind, placebo controlled studies on any HRT (for example where 50,000 women are given dummy tablets/ patches but don't know which they are using.)
Estrogen does not cause BC- this is well accepted now. There are fewer cases of BC in women using only estrogen compared to women not using any HRT.
It's the addition of the progestogen that may increase the risk and at the moment there is a 'tier' of risk with synthetic types being more risky.
The research is observational. women complete questionnaires on if they used HRT, type, duration etc. Much of the info used is from women diagnosed with BC then compared with women who never get BC.
The concern is that some of this information from women is not accurate or that they may have had undiagnosed BC at the time using HRT (some drs say it takes 10 years for a tumour to grow and be visible on screening.)
There is some research which followed women for 12-14 years where mainly micronised progesterone was used and it showed no increase in BC.
These were not studies into HRT as such but the women were being monitored for other diseases and some did use HRT.
It's unlikely there will ever be any proper studies because why would women volunteer to take HRT very long term IF they had no symptoms and IF it might cause harm?
Using HRT long term should be taken as part of your 'whole health' and that includes risk of heart disease, bone loss, and how much your symptoms affect sleep, quality of life etc. IMO all women should have a bone density scan (NHS refuse this unless you've already had a fracture or strong family history) because for women under 60 HRT is the preferred treatment (in the BMS guidance.) That means many women may use it for 10 years or more.
Not sleeping long term is possibly more detrimental to your health than HRT- risks of diabetes, heart disease, dementia - so it's balance of risk/ benefits you need to make.