Waiting for a BFN after miscarriage

Iv recently had a mmc and had a surgical evacuation to remove the pregnancy on Wednesday. From measurements it looks like my baby died around 2 weeks before the evac so I assume my hcg should of slowed down, stopped climbing or started to drop?? However it’s not been checked since very early in my pregnancy and the hospital were not keen to check it. They have advised me to take a pregnancy test 3 weeks following surgery and should have a bfn. However, at the epau last week she said should be bfn after 2 weeks following surgery. I am well aware that you can ovulate sometimes quite quickly again after a mc and don’t want to miss the window by waiting 3 weeks to test!! However POAS and seeing a bfp is just cruel when I know it’s just my left over hormones. I must say I don’t feel pregnant anymore unlike last week and feel like my hormones are crashing, my bleeding has practically stopped now I’m just having some cramps. I have begun bbt again the last 3 mornings. When should I start my opks again? Wait for the bfn?? I’m so confused 🤷🏼‍♀️ x