My dad is a tile guy. Corners always crack. Always caulk the corners (and the edge of the tile).
When you say caulk the corners, you mean instead of using grout, caulk them instead? I have cracked corners in my other bathroom, and trying to avoid that this time, and already was considering just caulking them.
Also, one of the 3 batches of concrete I made didn’t set, and it has been way too long for it not to have. Not sure what happened, as I was very careful to get the ratio right and did the exact same process on all 3 batches. All were the same, but one didn’t go right. I put a fan on it for the day, hopefully that works, or I’ll have a big mess on my hands. It just needs to be solid and level so I can tile!