Schluter Kerdi line linear drains go up to 1200mm long.
Put one full width of short side of shower against wall.
Then shower screen perpendicular to the drain, that way the drain is full width of shower.
Wedge boards will direct water downward towards drain.
They have sufficient fall in them for water drain off.
I suggest kerdi line because a tanking membrane comes attached to it.
Cover the rest of the shower floor with the kerdi Keba tanking and it'll be watertight.
As long as your fitter knows what he's doing!!!!!
And to say he's never used the materials or done a wet room, is a little concerning.
If he's good at his job he should be able to cope.
But make sure it's done exactly as instructed by manufacturers.
Failed wetrooms tend to cost a fortune to put right.
Maybe consider epoxy resin grout,at least in shower floor.
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