Shower floor, Tile over tile; laying to fall

My shower area has 300x300 ceramic tiles, done by the previous owner's contractors a few years ago - it has minor leaks through the pointing/grouting and was also not properly laid to fall (water ponds in some parts).

I have purchased new tiles and "Dunlop Tile on Tile" adhesive, which is meant for lay new tiles on exisiting ones. I don't want the hassle of hacking/removing those, and the existing tiles do provide a fantastic waterproof layer anyway (other than at the grouting). There is only one bathroom in this house, so I am aiming to start & finish the work on the same day.

I am thinking of applying silicone or acrylic waterproofing sealant at the existing grouting, then provide temporary "very thin strips" of increasing thickness mounting board as a base to "slope/fall" the new tiles, then apply the "Tile on Tile" adhesive, then bed the new tiles, and finally fill in new colour-matched grouting.

Does this sound workable & any suggestions or advice, particularly on how to build up a good & even fall (the water floortrap is a small square one somewhere in the middle of the rectangular shower area; the front of the shower area (including the bottom of the entrance door) has a 2 cm vertical aluminium strip, so there is some room to do a proper fall.

Many thanks in advance.