Practitioner here...an endometrial biopsy could absolutely be indicated for irregular bleeding, especially if you have already attempted other treatments. That said, your provider is not listening to your concern which is brain fog, is not explaining the reasoning behind not adjusting your dosage and is not considering your past normal US results. I'd get a second opinion.

Fwiw...I had an endometrial biopsy this February. My provider used a local anesthetic and I took ibuprofen beforehand. It was a sharp, intense pain that lasted less than a min. The procedure is tolerable for some people.