Was the reason for using the SLC, to level out the floor? If the floor is out of level it’s best to remove the floor completely, level it up by packing out the joist or levelling the joists themselves & then lay a new floor, usually 18-25mm WBP ply (not standard ply). A 30mm fall in 1500mm is excessive, is there a reason?
A 6mm ply over board is nowhere thick enough either; 15mm is the BS & most tile adhesive manufacturers recommended minimum but generally 12mm is sufficient in light load/use areas such as a bath/shower room. Just screwing into the top of the original floor boards is not ideal either; when over boarding, I always fix through into the joists below. You’re going to have a mish mash sandwich of 3 different substrates for a tile base & adding additional bits of 6mm ply at the low end to pack out the SLC is not something I would do or recommend either. SLC’s are intended to take up small discrepancies & provide a flat, level surface (3-10mm in this case) but I doubt they will add much to the floors overall rigidity. From a tiling viewpoint, I'm sorry to say it’s not the way I would do it & I’m in doubt as to whether it will provide a suitably flat or rigid tile base. If your tiled floor is to last, you need a rigid floor base, if it flexes outside the adhesive/grout tolerance range, it will fail.