I would request the extension and delay of the interview before any medical exam (which expires after 6 months so there’s really no point of completing one) and interview. However, as I mentioned, I doubt the consulate will extend it several times and it’s fully at their discretion whether or not they will grant you an extension. If you go this route be prepared to take the risk - which is them denying the extension and then you’ll have to to either bite the bullet and interview and get the visa and move or let it expire and file anew for another fiancé or spousal petition.