The reason you might be having trouble with flange because it might have a lead poured caul joint . Take off strainer and look, and take a knief are something and scrath around the joint and see. If it is take an el. drill and 3/8″ bit and drill a series of holes in the lead joint and finaly dig it out with screw driver,
. If you cant do that then go down below and take a sawall and metal cutting blade and cutpipe and use a nohub band and piece of pvc and reinstall, if that is to much for you call a Lic. Plumber.
Art retired plbg