Cover shaft 1.5 x 1.5 concrete like structure

Maybe someone will speak up here, otherwise I would have to call the [baustoffhändler] and ask what he thinks and hope that he just doesn’t want to sell o_O

I have now decided to make the slab out of concrete instead of wood. Are bearing pockets in the existing masonry absolutely necessary for this, or is it sufficient to have a connection reinforcement into the masonry with 8 mm bars every 15 cm and then simply pour the slab in between? The red parts are the 8 mm bars driven about 10 cm deep into the masonry. The light brown is the connection of the individual bars twisted together with 6 mm rods. Thanks in advance for your answers. A picture is included to illustrate what I mean for better understanding.