Shower wall structure

[ir50]

I'm replacing a bath with 1200 x 800 shower.
Old tiles are off walls. The wall that is the shorter side of the shower and will have the controls is ply upto about half height. The rest was plasterboard but came off with the tiles. The longer side wall is made of the pink soundproofing board with insulation on back so don't want to remove that. Tiles came off this quite clean- just need to clean left behind adhesive a bit.

So what do i do with shorter wall?

1. Remove ply and reboard in PB and then tank both walls?
2. Leave ply and fill in rest with ply or PB and tank both walls?

Is there really any point to using a backerboard rather than above if tanking anyway?

Any preference over the tanking matting eg homelux or the paint on tanking shower kit from Mapei apart from drying time

Thanks in advance

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[Dougs Third Go]

I'd personally rip it all out and start afresh. Plasterboard 12.5mm with a tanking membrane would suffice (as long as tiles aren't over 28kg/m2).

 

[Bubblecraft]

As above