Installing a shower niche. Bought a prefabricated "Ready for Tile Niche". Want to use stone shelves that I had custom cut for the niche. Will the tile be enough support for the shelves? They are 16 1/2" long x 3 5/8" deep x 3/4" thick so they are pretty solid and heavy. Tile is 9mm/0.35" thick.

First photo is inspiro.

Second photo is premade niche.

Third photo is shelf - bought winged for bottom and non-winged/flush for top.

Fourth photo is tile.

Do you think I need more support for the shelf? I can't seem to find anything that would fit the fat shelf and also stay subtle so if you have a suggestion, please let me know!