Officially, you should have the biopsy.
It may be difficult to get your prescriptions continued if you decline this, if someone decides to be a real b* about it.
However if it were me, or anyone I cared about, and it has stopped with an increased dose of progesterone, and the lining was only 2 MILLIMETRES above normative limits based on women without HRT, I would much rather just have a routine follow up ultrasound, as long as this wasn't a deal-breaker to get my prescription continued.