We have an old house, 112 years old. We redid our upstairs bathroom couple years ago and decided to have contractor install marble saddle transition between original maple floors to the new marble bathroom floor. We were not there for install but I'd say within 2 weeks of completion threshold cracked. Obviously done incorrectly. Installed on top of wood floor with thinset. Many gaps. Contractor gone, don't ever want to work with them again due to multiple issues. How to rectify this? I'd love to still do marble if possible. Do I need to cut the old maple flush, install proper subfloor for marble and go from there? The new bathroom floor is 3/8 higher than maple floors. Maple is 3/4 thick. We used 4 inch wide marble piece before. See pic. Our house is not level and has all the perks that goes along with being 100 year old house.