Wetroom Former Size brand and type

[One Day]

With the Wedi shower trays in particular the Fundo ligno, they recommend setting the waste in mortar by making a box between the joist, placing the waste in and filling with mortar.

Is this an essential part of the install?

They also make a pre-formed insert which kind of does the job for you. Takes the place of a mortar box, you just need to make sure it's supported below.
Yes, in either case it's essential because the potential flex at the drain (the thinnest point) can open up the connections on the drain.

 

[One Day]

It's called the "drain substructure element".

 

[YT233]

Thank you for your reply,

In regards to the sealing tape, I know wedi say the use of the join Silicon is enough. If you were to use the sealing tape also, does this need to be applied using the 520 adhesive?

 

[One Day]

Thank you for your reply,

In regards to the sealing tape, I know wedi say the use of the join Silicon is enough. If you were to use the sealing tape also, does this need to be applied using the 520 adhesive?

Yes it should be.
A sealing kit contains everything you need for one tray.
It's not a Silicon by the way. I think you mean Wedi 610?

If the area isn't prone to move (i.e. no timber involved) then I'd simply use 610 to join blue to blue (core to core).
But on timber I'd suggest using the sealing kit with tape and 520 adhesive. Safer in the long run.