My last last was a Reno, we had an open plan kitchen dinner that bridged the new and the old. We had the new slab made up to existing height. The old part was also concrete so that helped, then chucked SLC over the lot, then used Wunda over floor boards throughout, tiled straight on top. No issues. I was worried about the joint between new and old, but no problems. Probably waited about 6 months for it to settle before tiling. I think the wunda boards added a layer of decoupling to for some differential movement.