If at some point you are going to bake bread, you may want to use split firebrick>fireclay to level>Biscotto floor, otherwise for making pizza, 50mm of biscotto should be fine.
I'll think I'll skip to go full out to make it a bread oven, that ship has sailed. Rather an regular all-around oven and perhaps sometime just try out some kind of easier bread making or similar.
So right now the decision lies between two options; just the 50mm biscotto on top of the insulation board or cast a 15mm layer of homebrew and level the biscotto on top of this (after drying so it is replaceable).
The first option saves a lot of work, the second I think would be closed to what are field-proven dimensions. In addition it would in my case thermally connect the dome and floor as well as close off the cooking area from insulation board (refractory material feels somewhat sketchy despite the safety declarations) as well as an additional barrier for water to find its way in (based on that the home brew layer doesn't crack for certain).
Anyone believe the additional 15mm thermal mass in this case and thermal connection between the dome and floor are positive and worth the work?