Tiling large format slate onto anhydrite screed - Tile Advice

[simonwoody]

I am planning to lay 600mm x 600mm x 20mm slate on to an anhydrite screed. It is a very thick tile! I was not familiar with anhydrite screeds and have been reading some of the horror stories. I was set on using Anhyfix but I see Tilemaster have stopped producing it and I spoke to Ditra technical and they said they do not recommend gypsum adhesives at all.

So my current plan is the following:

1. Remove laitance (already done, the screed company came back to do it)
2. Two coats of Kerakoll Primer A Eco
3. Stick down Ditra 25 with Kerakoll H40 Advanced Adhesive Rapid Set applied with a 4mm trowel
4. Tape the joins between the Ditra 25 but not the edges of the room (except in the bathroom where I will tape the edges)
5. Use a 12mm trowel to apply Kerakoll H40 Advanced Adhesive Rapid Set on top of the Ditra 25 and use a 4mm trowel to butter the back of the slate tile.
6. Use Progress Prolevel Tile Levelling System to level the slate. (These are the only ones I could find that are tall enough to accommodate the 20mm thick slate)
7. Leave to dry for 7 days
8. Seal with Lithofin MN Stain Stop
9. Grout with Kerakoll Fugabella Color Resin-Cement High Flexibility Rapid Wall & Floor Grout
10. Leave another 7 days
11. Seal again with Lithofin MN Stain Stop

I had a couple of questions about the process:

1. Is it right that you need to use rapid set adhesives if fitting slate?
2. The slate is very thick. Is 12mm trowel for the floor and 4mm for the back butter correct?
3. Do you still have to leave for 7 days to dry before sealing if you have used rapid set?
4. Are there any disadvantages to using rapid set?

Any other advice would be greatly appreciated.

Thank you!