Advice re plasterboard skimming


I have a shower cubicle with plasterboard walls which I want to tank with the Bal WP1 waterproofing system, and then tile.
However before that, about 1 sq metre of the wall needs skimming to bring it up to the 3mm depth of the surrounding plaster as it was replaced because of water damage.
The problem is that Bal recommend that new plaster is left at least 4 weeks before tanking, but I cant wait that long. I asked them if I could skim the wall with one of their adhesives rather than plaster, but they advised that this wasnt recommended as the adhesive was only strong in shallow depths.
Is there a suitable quick drying alternative that I can use for the skim? Grateful for any advice.
I will be using Bal Grip adhesive and Bal Microflex grout for the ceramic tiles.
Cheers
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